AT&T officially confirmed that their Samsung Galaxy S4 will be getting the Android 4.4.2 KitKat rollout forthwith. The update has already started rolling out and will push out in waves OTA. The new version is firmware version KOTH49H.1337UCUFNB1, and the baseband version will be I337UCUFNB1. The update measures in at a hefty 414MB /or 842MB depending on which version of Android you currently have on your handset. Here's a quote with the full details,

Update the Software for the Samsung Galaxy S 4 (i337)
Update summary

Effective February 26th, 2014, a software update for the Samsung Galaxy S 4 will be available to AT&T customers. This software package updates your phone to Android 4.4, and includes multiple improvements. The update file size varies between 412MB - 842MB depending on your OS version. The update is available to download over Wi-Fi only.

What's changing?

Android 4.4 OS upgrade (Android - 4.4 KitKat)
Samsung Galaxy Gear compatibility
Drag and drop to uninstall or disable applications
Call reliability improvements
What to expect after the update

Although there should be no impacts to settings or data, we recommend that you back up your media files to an SD card, a PC, or using a favorite application (from the Play Store), prior to upgrading the software. For information on backing up and restoring your phone using Samsung Kies, click here.

How to update

Customers will receive an alert and be prompted to download the update, or have the option to initiate the update manually. When prompted, you can postpone the update up to 3 times, and 4 hours per occurrence. After the update has been postponed for the maximum allowable time, your device will automatically download and install the update.

To initiate the update manually, connect to a Wi-Fi network.
Drag down the notification bar, and then tap > from the "More" tab, scroll to and tap About device > Software update > Check for updates.
Tap OK.
Tap Continue.
The device will automatically start downloading the software update. Once complete, tap Continue and the device will update and restart.
Please wait until your phone reboots completely, and an "Update complete" message displays, and then select Done.
How to verify the update

Drag down the notification bar, and then tap Settings .
From the "More" tab, tap About device.
The following information will be displayed:
Android version: 4.4.2
Baseband version: I337UCUFNB1
Build number: KOTH49H.1337UCUFNB1
Source: AT&T