AT&T and Samsung aren’t yet pushing out the Android 4.4 KitKat update for the Galaxy S4, but you can get a taste of it if you’re adventurous and willing to play with a leaked build. The software update was recently published on the XDA developers forums with the I337UCUFMLD build number, though SamMobile notes that’s an older build than the one that was leaked for the international variant of the phone recently.
In other words, that means AT&T is probably testing a newer version of the software, for whatever reason. You should keep in mind that AT&T will publish the update when it’s ready, so it hasn’t pushed this one for one reason or another, likely because it didn’t meet the carrier’s test requirements. That said, if you want a taste of what Android 4.4 KitKat has to offer, and you’re willing to fight away any bugs that might still exist, the software update is out there for you to wrangle with.
If you’re feeling the KitKat itch, be sure to backup your phone first and know that you’re applying the software at your own risk, and at the risk of bricking your smartphone or voiding your warranty. Good luck!