New Features in 4.0:
-Updated User Interface with Tabs on top (Safari 4 beta, Google Chrome style)
-New Add-ons manager
-WebM video support (HD quality video)
-Amped up HTML 5, Privacy, and CSS support.
-Sync all your passwords, bookmarks, and settings across devices
-Speed Improvements (Probably something like iPhone 3G to iPhone 3G in terms of web browser speed)
And the best part… That flashy new UI is not currently available in the current Mac and Linux beta release so stay tuned.