There's two things about this, at least for me but if you think about it, maybe for you too.
1.) There is NO compelling reason to go to vB4. None at all. It's not demonstrably better, there's nothing about it that just pops and makes you say "gotta have it."
2.) Once you "upgrade" to vB4, it is a major undertaking to ever go back.
I am never, ever giving up my 3.8.x, they will have to pry it from my cold, dead hands.
That's the spirit!
I fully understand you.
I think that some interesting summary of the things can be found in wikipedia -> http://en.wikipedia.org/wiki/VBulletin . After reading this article it's getting much clearer what differs in vb3 and vb4 and who developed vb3 and who is developing vb4. As per user experience typing in Google "vb3 vs vb4 vBulletin" is the way to get an idea.