The major of features of vBulletin 4 that you'll notice is;
- Profile Pages have a different layout, with a Visitors message feature.
- Thread tagging and Tagging Clouds
- User-Customisable Profile Pages
- Post Edit History
- Notices System
- New, up to date Theme
- Few more inline moderation tools
These are only the default feautures, some of these may be turned off, we may have other mods that are useful, the ETA on vBulletin 4 could be described as close.