I downloaded the trial and found no discernible difference in performance between the two. Mind you, I don't play games in my virtual machines or do anything too intensive... mostly I use it for Outlook. So I'll be sticking with v. 3
I tried the demo and found Fusion 4 to be a little more refined than 3, but I noticed no appreciable difference in speed. However, I notice a tremendous difference when I downloaded and used Parallels 7. Loading a Windows 2007 Virtual Machine and OneNote program took 85 seconds in VMware and only 20 seconds in Parallels. I have officially switched to Parallels after 4 years of using VMware Fusion.