Four Gigabytes is a lot of RAM for a workstation or gaming machine, but the industry has pretty much forced us into using this much (especially if you're a heavy user of Virtual PC). Most applications today are 32-bit and only address 2GB of RAM at a time (like Photoshop), but we need more memory for all the other running goodies that are memory-starved. Even though Vista runs on 1GB of RAM, 2GB is alot better, and 4GB will unleash its potential.
Unfortunately, users of the 32-bit flavors of Vista have to run through some hoops to see all of the new RAM they've upgraded to. Even then, they may not see all of the RAM available because of various hardware issues. This article addresses various methods so you can utilize that massive amount of memory.