The 719MB update enables the GS 4 to work with the Galaxy Gear smartphone. You can check for the update for an update (from About in the Settings menu).
Update: 11/15/ 1:31PM: There have been reports that the update starts and then stops in the middle of the process.
I contacted PR reps from AT&T. As far as we know, there has been no official word from AT&T about the Samsung Galaxy S4 update. Usually they receive notification of updates in advance.
There are forum members as well as commenters (see below) who state: "So after waiting for 3.5 hours to use my phone... the software update promptly got to about 60-65% and suddenly has disappeared from my phone. Going onto my software update area says "Phone up to date! Check again in 20 hours and blah blah minutes") despite still being on 4.2.2."
It seems like some who try the update, get stopped in the middle.
The update is a two step process, involving a download of the update to your phone, followed by the installation of the update to your phone. During the installation step, your phone will be unable to make or receive calls.
Prior to download/installation, you will be reminded with a message stating "Your device will be out of service while installing the download”.
Your battery should be charged to approximately 50% before starting the update and should not be removed during the update. Updates are available OTA (over the air) only and are 'pushed' to devices in stages over a 3 week time period . Device selection is random.