The AT&T Samsung Galaxy S4 Android 4.3 update has now been released, the update is around 719MB in size and should be available for your Samsung Galaxy S4 on AT&T as of Now.
The update brings a wide range of new features to the Samsung Galaxy S4 in AT&T, which include support for the Samsung Galaxy Gear and also Samsung’s new security suite KNOX.
You can see a list of what is included in the AT&T Samsung Galaxy S4 Android 4.3 update below.
OpenGL 3.0 Support
GALAXY GEAR Support
TRIM Support (Makes the device much faster)
Samsung KNOX Implementation (KNOX bootloader and dedicated application)
Samsung Wallet comes pre-loaded
Improved RAM management
Improved Display colour reproduction (Display looks much sharper than before)
Improved TouchWiz Launcher (Much less launcher redraws and less lag)
New Samsung Keyboard
New Samsung Browser (Full screen by default, new tab interface and more)
New Reading Mode (Optimises display for reading, used by only a few specific applications)
New Camera firmware
Minor UI tweaks (Contacts app, Flashlight Widget, dialog boxes etc)
Source Android Central