is there something that needs more than the other 2 gigs?
(pssst the answer would be no, not in normal usage)
Yes and no.
Most common programs won't use more then 2GB. 64 bit programs on the other hand tend to be optimized for large amounts of memory, specifically their not frugal with memory use. Put a few programs together and you'll shoot past 2GB pretty easily. Also windows file cache must be taken into account, maxing your physical memory usage will result in small caches and frequent disk hits even for random background process's.
Memory is so cheap now, just get 8GB and be done with it. Will go a very long way.