I have tried to move all of my blogs over from MT 3.34 to MT 4 and have been fighting for a week now as I can not successfully rebuild my archive pages. I am not some newbie with this having been using Movable Type for three years now so here are my recommendations.
I am right now still struggling with this and have tried everything from reloading my cgi-bin and static files to even recreating a new database and trying to import a blog into a brand new database but still get 500 errors whenever I try to rebuild my archives page. I have been to the movable type forum but the answer that one user gave me did not work although getting rid of bad plugins was a great idea that I had not though of.
1. Setup a new database and do some testing, make a new blog in a new subfolder with a fresh release of MT 4.0 and import your entries and copy your templates over. Check your performance.
2. Take this chance to look at your code and make sure that everything is setup properly in all of your templates. Keep your pages as lean as possible just to make everything work better for your users.
3. Try all of the different browsers and test your code? Do the pages render properly, or just even good enough?
After you have done these test which should only take a couple hours at most then you will feel a lot more confident going forward. The upgrade to 4.0 only takes about 10-20 minutes and that is mostly all of the files copying over FTP.