This has a nice breakdown of the new features:
Samsung Galaxy S 4: Bigger, thinner, smarter
Then there's Samsung's official Galaxy S4 webpage.
I do love my S3. But it's interesting to see what features they're developing, where we're going now and in the future. But having just gotten my S3, I'm not making a change just yet. Still, it should be interesting to see where they are when I'm eligible for best upgrade pricing (at T-Mobile you can upgrade at any time, you just get to a point in your contract where the pricing is more otpimal). I think I'll stick with Samsung if they continue to do this well.