[Gio~Logist]

With vB5, you can still always manage a vB4 website. You can always use versions lower than that of what you paid for.

vB5 isn't as easy to customize for and also is far different of a user experience than what users are use to. It's also completely different code than what earlier versions used, making a lot of developers turn their head. All in all, it's a little stiff - to say the least. I've modded it a few times for clients. Although completely possible, it wasn't a pleasant experience.

As for the CMS, I was never a fam of the CMS in either system. vB5 has a responsive design for articles and what not, making it show just fine on mobile, so that's cool. However, like you said - not that many customization features. All In all, I'd suggest a custom homepage or CMS using vB4, and working on making that responsive. The other alternative of course would be using wordpress for your homepage and cms and then vB strictly for forums.