Spent a few hours, finally upgraded to v4. Some features are nice but the parallels runs slower than before (v3) and it's much more memory hungry. I allocated 1gb for it and it literally occupied that much even when I didn't run much. Worse, it wouldn't release the memory after I pause it, contrary to what the help page said. I think this explains why both windows and hosting OSx become slower after the upgrade (I only have 3gb memory total).