Apple says it supports 2GB because *officially* that's all it takes. The reality is that the chipset, the machine, and the software supports 3GB. The reason Apple says it takes 2 is to keep it farther separated (artificially, mind you) in performance from the MBP, so they can justify the price delta between the two models.
4GB won't work in your machine, as far as I know; I haven't tested it, but I'm sure someone has. It'll do one of 3 things:
-See 3GB of the 4
-See 2GB of the 4
-Refuse to boot until you take it down to 3GB or less
It's an odd number, I know, but for whatever reason Intel designed the i945 mobile chipsets to only address 3GB. The funny thing is, it can see the 2GB stick whether it's in slot 1 or slot 2, so you'd think that it'd be able to see a 2gb stick in both at the same time. But somehow, for some reason that is totally beyond my comprehension, it won't.