AT&T is rolling out Android 4.4 KitKat to yet another device in its roster today, in this case the rugged variant of the Samsung Galaxy S4, the Galaxy S4 Active. The update is currently being pushed out to the device with all the usual features and improvements that are part of KitKat, along with changes introduced by Samsung.
These include faster performance, reduced memory usage, white status bar icons, new lockscreen with camera shortcut and full-screen album art, ability to select the default SMS and home launcher apps from settings, advanced location controls, wireless printing support, new sound controls for specific Samsung apps, extended emojis, call reliability improvements, and general stability updates.
The update size will vary between 447MB – 877MB depending on the current version of your OS, and the rollout will be limited to Wi-Fi only, at least for the first few days. Expect it to show up on your device within a week or so in the usual phased manner, though you can also do a manual check in the Settings » General » About phone » Software update menu.