If you use the MultiBlog plugin by David Rayners and you upgrade to MT4, when you go to rebuild your site, you may get the following error message:
"Publish error in template 'Main Index': No handler exists for tag MultiBlogEntries"
The MultiBlog functionality is now part of MT4. All you have to do to update your templates is to change every instance of the tags MTMultiBlogEntries or MTMulitBlog to MTEntries and rebuild your templates.
So <MTMultiBlogEntries include_blogs="2,5" lastn="10"> and </MTMultiBlogEntries> become <MTEntries include_blogs="2,5" lastn="10"> and </MTEntries>.