T-Mobile only recently started pushing out the Android 4.4.2 KitKat update to the Galaxy Note 3, and now the uncarrier is extending availability of the latest version of Android to the Galaxy S4. The update is beginning to roll out over the air and through Samsung Kies with build number M919UVUFNB4, with the former sized at 388 MB. Your phone will have to be on build M919UVUEMK2, the last Android 4.3 build, to be able to get the update; as always, the rollout will be staggered, so you might have to wait for a few days before the update shows up on your Galaxy S4.
The KitKat update brings improved stability, and major features include white status bar icons, improved performance, reduced memory usage, improvements to Samsung's onscreen keyboard in landscape mode, inbuilt wireless printing support, ability to select the default launcher and messaging app from the settings menu, a camera shortcut and full-screen album art on the lock screen, and new emojis. You might also find that most apps are lesser access to the external storage than before, which is by Google's design and hence something you'll have to live with.
