AT&T users are receiving the update over the air (OTA) and the update is reportedly the same size as the Sprint update. However it seems that KNOX is not included for AT&T users even though they have all the other features.
Meanwhile KNOX is available for the Verizon variant of the device but the update can only be downloaded through the Samsung Kies suite. Additionally Verizon users are receiving the full system image instead of an incremental update which has resulted in a somewhat large 1.8GB download. The Verizon variant of the Galaxy S4 seems to have Knox included along with all the other features.
The likely explanation is that the AT&T variant of the Galaxy S4 is extremely similar to the International model which received the update last month but didn't include KNOX in its release. However we think Samsung will be quick to rectify this, since customers have been unhappy with the exclusion of KNOX and the inability to use their Galaxy S4 units in their companies.
Do you own a Galaxy S4 on Sprint, AT&T or Verizon? Have you received the update yet?
source - Droid Life | Android Central