Wow!! I am surprised by what is missing here....
I have used SPS forever and can't imagine using anything else. But here are a couple of points not mentioned:
1. you don't have to use the peg. I use Tuffboard cake boards (the corrugated boards). The peg is NOT going through that plastic!! I use a small hacksaw and remove it. I secure the cake to the board with double stick carpet tape. It ain't going nowhere!!!!!
2. regarding cake height: I have never worried about the height of my cake tier (and yes, you do need to include the board in your calculations).
I center the SPS plate over the tier bottom side down. I press the plate into the cake. I now have the impressions that tell me where to place the cake tier. I then use a sewing gauge; you know the small 6" slide ruler found in the sewing department. I insert that into one of the 'hole' impressions. I now have the accurate height of my cake tier. I then add in the height of the cake board.
3. I use this measurement as the guide for the height of the SPS legs. I subtract about an 1/8th of an inch (or approximately the height of the cake board). This insures virtually no gap will appear. I mark each leg at this height.
Now here's the kicker: there are 9" scored SPS legs that can be cut to the height that you need!! Use a hacksaw or if you have a scroll saw, even a Ginsu knife works. You are not confined to the use of the 4 inch legs.
I make a lot of sculpted cakes and seldom is a tier exactly 4 inches, but I use SPS successfully all the time. Don't give up. You just have to find the method that works best for you.
Yes... I have the 9" legs too (I have them because I was going to have to do a raised tier but she then changed her mind), but most are trying out the SPS for the ease of not having to cut the legs. The whole reason to use the SPS over the witon plates and pillars is that little peg that keeps the board from moving. I know carpet tape negates the need for that, but you might as well just use the wilton plates and hollow plastic dowels if you are doing that no?
Not trying to say your method is wrong, because, let's face it, there really is no "wrong" way to do it so long as it works right? 
This is just for dialogue that might be helpful in the long run.
Yes. I love that SPS is assemble and go. If you do it right, there is no need for hacking and tape and cutting and anything else. I don't understand why people want to take a system so carefully and thoughtfully developed to be easy and foolproof....and make it complicated. I dunno...to each his own I guess.
Leahs has rescinded her "listen for the swoosh" comment I believe. And foam core users, (me!), it is failproof as far as getting the peg lined up and in it's spot. if you do it right. Failproof. If you need some tips, ask me. I've got it down to a fine science, for foam core. Pretty simple too.
Okay, Jamie. Fire away.
I use foam core covered with press & seal under all my cakes. I think my main problems are getting the cake tier onto the peg without messing up my buttercream & resolving the gap issue. What kind of spatula do you use to place the cake tiers?
You DON'T add the foam core under ea cake into your total 4 inches, right?
If you've already posted this somewhere else, can you direct me to that post?
THX!!
BTW, here's the my 1st SPS cake:
URL: http://www.cakecentral.com/cake-photo_1548227.html
As you can see, I took the pic a little farther away. 
I must say though, I had some nice, tall tiers.
I think my main problems are getting the cake tier onto the peg without messing up my buttercream & resolving the gap issue. What kind of spatula do you use to place the cake tiers?
You DON'T add the foam core under ea cake into your total 4 inches, right?
If you've already posted this somewhere else, can you direct me to that post?
THX!!
Yay! A taker!
Lol...ok, here goes.
First, you want to trace your foam core specifically to each SPS plate. Meaning, take the plate turn it upside down, push on it, so it jams the peg into foam core. Hold your hand over it, and trace your circle. Cut out. You now have an absolutely perfect centered hole for your foam circle. Flip it over...cover as usual.
Assembly time:
I absolutely chill each and every tier prior to stacking,period, SPS< straws, whatever. A cold cake handles better, there's just no arguing this. Nofinger dents, spatula marks, etc. Follow Leahs instructions for making the mark before putting the legs on, put legs on, push the whole thing into the tier.
Now-with a CHILLED tier, pop that sucker on. Since you know your circle and board are exactly the same siz, you can eyeball until the peg grabs on to the perfect place. Might have to adjust a litte, but since your tier is chilled, you aren't dinging the icing up anyways.
*******The circle/cardboard/whatever absolutely under no circumstances ever under the sun, counts in the 4 inches we talk about. It's impossible. Your cake, from the circle to the underside of the SPS plate above it, must be the 4". More like 4 1/16 inches, but whatever.*****
Spatula...I don't even bother with it usually. I plop it on. But, remember, I'm working with chilled tiers. They can handle the rough treatment. IF I need to move it, if I've missed the peg, I take a little offset icing spatula and gently nudge it around until I've fixed it.
I used SPS for the first time ever yesterday for a New Year Celebration cake for our family get together today.
I used it with a stacked 6" and 9" cake, and was delighted with the ease of handling and set up. I actually have some atrophy and parallysis in my hands and I could do this quite easily.
I keep thinking, "Where was this for the 1000's of stacked cakes that I made in my 40+ years in the bakery"?
The time I could have saved from measuring and cutting dowels alone amazes me.
Okay Jamie, I did trace the SPS plate onto my foam core (peg down), cut out, & cover.
My cake was a little smaller than my foam core board (I use Magic Line pans). I place the cake on a bigger flexible board to ice. When I ice with bc, the buttercream has to be thick on the sides enough to cover the foam board.
I chill my cakes in the freezer for @ least 15mins. Is that long enough?
When I take the cake out of the freezer, I use a spatula to pry the cake off the bigger board & place it on the lower tier.
Problem here is that I have icing that's not supported by anything & it gets messed up when I pick up the cake & place it on the lower tier.
Oh, I think I need a class in cake stacking!!
Did your atavar just change?
I used SPS for the first time ever yesterday for a New Year Celebration cake for our family get together today.
I used it with a stacked 6" and 9" cake, and was delighted with the ease of handling and set up. I actually have some atrophy and parallysis in my hands and I could do this quite easily.
GEE, thanks! 
Okay Jamie, I did trace the SPS plate onto my foam core (peg down), cut out, & cover.
My cake was a little smaller than my foam core board (I use Magic Line pans). I place the cake on a bigger flexible board to ice. When I ice with bc, the buttercream has to be thick on the sides enough to cover the foam board.
I chill my cakes in the freezer for @ least 15mins. Is that long enough?
When I take the cake out of the freezer, I use a spatula to pry the cake off the bigger board & place it on the lower tier.
Problem here is that I have icing that's not supported by anything & it gets messed up when I pick up the cake & place it on the lower tier.
Oh, I think I need a class in cake stacking!!
Did your atavar just change?
My foam core is always bigger than my cake, or else cake would show through. You do what I do too....with the bigger board to hide the foam core when I don't want to use a border. Hmmmmmmm. I think you need to chill longer. I put them in the fridge and go do something else for about an hour or so. They are very firm before I touch them again.
And yep---a pic of my son from this past week. This was such a good picture of him!
Yes. I love that SPS is assemble and go. If you do it right, there is no need for hacking and tape and cutting and anything else. I don't understand why people want to take a system so carefully and thoughtfully developed to be easy and foolproof....and make it complicated. I dunno...to each his own I guess.
I think you missed the part where I said that I do a lot of sculpted cakes that are rarely exactly 4 inches. Yes, SPS was 'carefully and thoughtfully developed to be easy and foolproof' but that doesn't mean that it works perfectly as is for every application.
I was introduced to SPS by B. Keith Ryder. One of the first things he told me was to remove the cap from around each leg receptacle on the plates. I think you mentioned in a previous post that you also do this. I think that means that you've made an adjustment to the system as well.
My point is that what works for me may not be ideal for someone else's application. I don't use cardboard cake circles. The peg is not going through the plastic board. The double stick tapes works fine for me. I'm not asking anyone to do this. I just offered it as an alternative. So sorry if this seems complicated.
You are supposed to remove the caps if your tiers are sitting flush. If you don't, you could have a big ole' gap (and your icing and cake will be compressed around the legs)... not exactly adjusting the system, but I completely understand using it the way it works for you, and carved cakes are a whole other ball game so I also completely understand having to modify the system for something like that.
When I measure the tiers for height I always start at the base (usually the turntable or an upside cown cake pan since I use those on top of my turntable to increase the size of my turntable for large tiers). So I have to add the height of the cakeboard to my final cake height to get the right height. If you are measuring up from the top of the cake board then you wouldn't want to add any extra height to your 4".
I think you need to chill longer too. Is your icing made with shortening? If so then you will have to chill a long while before it will be really solid. Jamie and I both use SMBC which is mostly butter so when it gets cold... it gets rock solid. Seriously... you could juggle a few small tiers with little marring. 
Chill them for 30 minutes (since you freeze them) and see if you can still mark up the sides with finger prints... if you can.. go longer.
I cover all of my cakes with fondant so I don't have to worry about the buttercream hiding the board since the fondant will cover it, but you will definitely need to chill for a long while to assure you won't mark the BC "lip". Use a large spatula... you might want to look into getting a cake lifter.
I think you need to chill longer too. Is your icing made with shortening? If so then you will have to chill a long while before it will be really solid. Jamie and I both use SMBC which is mostly butter so when it gets cold... it gets rock solid. Seriously... you could juggle a few small tiers with little marring. Chill them for 30 minutes (since you freeze them) and see if you can still mark up the sides with finger prints... if you can.. go longer.
My BC is 5c hi ratio shortening with 1/2c butter. I guess I was sort of afraid that it would cause some condensation (as it re-thawed) if I kept it in the freezer for more than a few min.
I cover all of my cakes with fondant so I don't have to worry about the buttercream hiding the board since the fondant will cover it, but you will definitely need to chill for a long while to assure you won't mark the BC "lip". Use a large spatula... you might want to look into getting a cake lifter.
I actually have 2 of the Wilton cake lifters.
I really only get condensation in the summer, but I live in the northeast. Not sure where you are, so you could have what I call summer all the time... LOL. Another thought since you are using a powdered sugar based icing, would be to do your final smoothing when they are stacked.
Take a peek at Enda's tutorial here...
I think you should be able to use her techniques with your icing and smooth out any bumps you might get when stacking.
I just watched tonedna's stacking video and I am wondering why she used straws for support for the last tier instead of using "cardboard dowels" like she did in the rest of the cake. Any clues?
Also I have been using the SPS system since last summer and I too have the problem with too much space between the bottom cake and the top cake. 99% of the time I have to cut my columns down because my cakes are usually just shy of 4". I feel like even if your SPS cake is flush with the bottom cake, it still adds height that needs to be covered with a border. That is the one thing I don't like about it.
I think people that cover cakes with fondant have an easier time with it. I only cover cakes with buttercream.
I use SPS all the time and I rarely use fondant. First and foremost, you MUST make sure your cakes are tall enough. You don't want to have to
use gobs of icing to make up for the gap....but in a pinch it'll work. 
While I'm stacking and icing, I have an SPS dowel right next to me so that I can make sure my cake will be tall enough. It helps tremendously to see before I get all done icing whether I'll be short on cake or too tall.
Once I get ready to put the dowels and the plate on, I scoop just a little bit of icing out from the place where the plate will be. That way, SPS plate will sit just below the cake surface. When the next cake goes on, CAKE is level with the surface, not the SPS plate.
It took me a little while to get used to it, but now I wouldn't use anything else!! 
Why is it when I fill my pans 2/3 full I still don't get a 4" cake? The batter reaches above the top of the pan during baking, but then shrinks in height as it cools. Do you guys fill your pans full?
No! Do not fill pans all the way! You will end up with a mess.
You don't need the cake itself to be 4". If it were, you'd have no room for the filling and icing.
It's math. And I'm a dumbass with math. You have 4 inches or a little over, more like 4 1/6th to 4 1/8 inches to play with. Cake should be 3 and 1/2 to 3 and 1/4 inches of that total 4. The rest is divided among filling and frosting. It's that simple.
I keep a ruler next to me while I am assembling. I measure the cakes themselves, and start subtracting from there.
My cakes don't sink very much when they cool. Maybe a little...but it's not significant. Have you tried using the bake even strips? Those help if you have Wilton or Magic Line pans.
I'm math-challanged, too! I just set my agbay right at 2". If that's too tall then I know I'm going to have to make up the remaining height in filling and icing.
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