[Wayne Luke]

It will not be backwards compatible with templates or template modifications. The entire style it being rewritten from the ground up. Any modification that uses templates will need to be rewritten to use the new variable registration system and template classes. The templates used by modifications will have to be rewritten to use the new template syntax. Any modification that rewrites templates in the cache or during output will most likely fail as well. None of the current 3.X styles will work in vBulletin 4 and will need to be recreated from the beginning.

If your modification doesn't access templates in anyway or create any onscreen output than it will most likely still work.