I have an upcoming wedding cake that has 4" round top tier. I am not finding that size in the SPS system. What have people done in the past that have had a 4" top tier but wanted to use the SPS system? 

A 4" round is such a light weight cake it really dosen't need any support.  Just place it on the other tier and the pipe the border.  That will hold it just fine.

That's what I was originally thinking but there is going to be a topper on it. It is a rather lightweight topper for being wood. I wouldn't add the topper until I got to the venue of course. I guess I am just paranoid since the delivery is in a touristy area and traffic can be a nightmare.

I've been known to put dowels in the tier below the 4" to support it, even though I'm the biggest SPS advocate!  Anyway, I stopped using a 4" topper, since commercial wedding cake toppers are sized for 6" top tiers anyway.

I think I may just do that. I have the thin bamboo ones that I could use. I am totally kicking myself for suggesting a 4" top tier. They aren't keeping it, so it was going towards their guest slice count. It was so difficult explaining to her why the topper she wanted to order wouldn't fit. I just may make it a rule that none of the top tiers can be smaller then 6" unless they aren't putting a topper. Thanks for the responses!