I'm using LR4 with Win7-64. No issues. Just kinda' slow
This is what's interesting; some people are having speed issues, some are not. My quad core and Win 7 seems to have no speed issues - however I just came from a seven year old dual core, so anything would be faster.
LR 4 speed is a common topic on other forums, but it seems hit and miss. I can't speculate as to why as I don't know enough about computers, but it's talked about quite a bit.