I've been using a lot of this in a developer preview for a while now, and the improvements over 3 are many, especially in the massively improved web inspector. The majority of the new things today are in the UI -- the tabs at the top, the top sites home screen -- and the majority of that is just Chrome done better.
I'm surprised at how quickly they've aped Google, actually. Perhaps just to spite them for taking so goddamn long over Mac Chrome? To be honest, I'd rather they'd aped the process-per-tab thing more.
Still missing: undo last tab close. Don't like the fixed "Add bookmark" button much, either.