It looks like Samsung's Galaxy S4 roll-out will start with AT&T in the U.S.
The carrier announced this morning that it will offer the Galaxy S IV for $250 with a two-year contract. Pre-orders begin on April 16 -- unfortunately there's no further information when you'll actually get your hands on it.
Given that T-Mobile plans to offer the Galaxy S IV on May 1, I expect AT&T will make its version available on or before then.
The Galaxy S IV packs in the usual upgrades -- a faster processor, more RAM, better camera -- while sharing much of the Galaxy S III's design. I found it incredibly comfortable for a 5-inch phone during my hands-on, though it's a shame that Samsung just can't get away from cheap plastic cases.