Well,
vBulletin 4 is quite a big change from vBulletin 3 and many people see vB4 as a rip of IPB like the multiple css work you have to do and the basic flow of the design like in the default skin, its pretty horrible in some factors.
Although, i don't have any issue(s) with it cause i code for both versions of vBulletin
There is some highlights and non-s that people like about it, i believe its more than my two cents but that's a basic gist.
Hope that helps in a way
-Baraka