I have to do a wedding cake for 80 people - I was going to do a 10", 8" and 6" but all the charts I found say this will leave me short a few pieces..is that right? I was debating making a 4" (out of a jumbo muffin tin) to make up for the six or so pieces I would be short. The person ordering the cake doesn't seem to care if it's enough cake for everyone since they figure some people won't want any..

I'm just worried that the 4" would look dumb, but don't want to be too short.

It's not often one can come exactly to the # of the servings needed.
Inform the customer you can offer her one that serves 75 or the next closest you can come is 101servings (12, 9, 6). Let the bride choose. In my opinion the 12,9,6 will look much nicer also.
OR you can make a 6" side cake.
Most people don't mind having extra cake. But......be *sure* you charge for the amount of servings they get not what they want! So if you go w/adding a 6" charge for the 87 servings they get; if they go w/the larger cake charge for all 101 servings.
BTW: I don't charge for the top 6" tier - that's for the couple to save so a 10, 8 cake would only give you 64; and the 12, 9 = 79

I would make the originally planned cake and offer a small brownie sized pan sheet cake to be cut in the kitchen for the event. This will make up for your missing servings, not cost you very much and yet cover the Bride...just in case everyone does want a piece.

The cake is actually being given to the bride sort of as a gift - I'm doing it very cheap as a favour my sister (the bride's sister in law) ( for $100).

I don't have a 12" pan, so I was hoping I could get away with not having to buy a pan right now, but don't have a problem getting one. I don't believe they intend to save any of the cake, so all the tiers would be included in the serving numbers - my sister was originally just going to have me make enough to serve 50 people and then she was going to make sheet cakes herself. I told her not to bother since a tiered cake for 50 would be just as much work for me as one for 80 and this way everyone gets the same flavour cake (she's not even really a hobby baker so I have no idea what her's would taste like).

There's nothing wrong with using a 4" cake for the top tier as long as it goes with the design and you have enough surface room on the top tier to decorate it the way you want. The only think with these smaller cakes is sometimes the proportions look a little off (ie. the look taller because they don't have as big a diameter- you could always make these slightly shorter so it flows better). What I would do is stack my pans and see if I like the look and then figure out if I would have enough room for whatever I'm putting on the top (if anything at all).

The other option is to stick with the 10/8/6 and stack that and make another 6" (or whatever size you need to accomodate the servings) and keep it in the kitchen and cut and serve. If you do it this way just make sure you fill/ice the same way as the tiered cake (no decorations beyond this are necessary).

Maybe you can look on-line for some 10/8/6/4 combo's to see if you like the look.

For my Winnie the Pooh cake I had used a 4" cake for the top tier (cake was 10/7/4).