Yeah and then got fed up of being treated like a BETA tester for a product which was inadequate for release and always seemed to have a shed load of bugs, which never got dealt with. 4.x has broken more times then a chocolate fireguard has melted.
A few exploits, the time the CMS (where it wasn't installed) broke profiles and other areas of the sites, non-compatibility with older themes - general pain in the butt to theme with it's new CSS and variable methods.
3.x had my support now IPB has it but VB and IPB have always been in different leagues.
Did you know vB4 is a code development of vB3? Not once has vBulletin, since it was brought out years ago, being recoded from scratch.