If you're happy on 3.8.5 and your members too I'd recommend to wait for upgrading. VB4 isn't yet stable and completely hasslefree like vb3 was and still is. You can expect to get security upgrades for the next 1-2 years. But it depends on your member counts, used addons, stylings to give you a more detailed answer and I think your question would be better asked over on vbulletin.com.
E upgraded hers from 3 to 4 and never looked back. It is much more admin friendly and you can do some amazing things with it. It bascially will replace the Wordpress+VB incompat software combo so many people are running. The blogging side of vb4 is excellent.
> forum continued
It was uninterupted and the new look-n-feel that translates from the blog/published side to the forum is seamless. No more of the, "gee, he just bolted on a 3rd party forum" mess that makes your forum look like the odd-man-out on your site.