I have 2GB of ram, Macbook 2.16GHZ Intel Core 2 Duo I was running Parallels 3 and i thought it was fine except for an annoying message that came up after waking from the screen saver. I decided to take the "deal" and upgrade to 4. Paid my $40 Big Mistake. The computer slowed to a crawl. I followed all the various suggestions and instructions throughout the forums (uninstall and reinstall parallel tools, tweak memory allocations, etc...). Both my XP side and my Mac side were totally unresponsively slow. I uninstalled and reinstalled 3 and it's working as I like for it to work. I did waste $40, and i have no idea if I can get a refund or just sell my activation key to someone. Who knows??! The bottom line lesson is this: If something ain't broke, DONT TOUCH IT!
Regarding the refund, please check this thread. But first I would suggest you to create a support ticket at http://www.parallels.com/support/desktop/ and put here its ID -- we could establish a remote session and try fixing the issue right on your Mac.
breadman - Did you try completely uninstalling 3.0, restarting, then installing JUST 4.0? That's what I essentially ended up having to do to go from 3.0 to 4.0.