Well, the bizarre truth is that 64-bit uses more memory than 32-bit, so while 4GB in 32-bit is awesome, for 64-bit it's a bit under used. DCS World was freaking out when it tried to use more than 2GB in my 32-bit Win 7, despite having 3.5GB available, so I went 64 bit. Ah, now I have the full 4GB to use, and DCS can use more than 2!
Except now DCS regularly used more than 2GB and the rest of Windows did as well (instead of sitting around using 1GB of RAM Windows now seemed to need 2!) resulting in painful disk swapping moments mid-mission.
I had to go 8GB to straighten that out.
So, if you only have 4GB, I'd say 32-bit isn't a bad idea. If you need to go 64-bit, though, go to at least 6GB ASAP.
The Jedi Master