Android Lollipop isn’t here yet, though Samsung is certainly working on bringing it to its flagship devices. In the meanwhile, those using the Galaxy S4 on AT&T can now update their device to Android 4.4.4, the last and final version of KitKat (and of Android 4.x.) The update is rather minor, with the only big change being the addition of KNOX 2.0 and a few AT&T apps (Keeper, Mail, Live, Uber, Remote Support, and Device Help) that you will probably never use (well, except Uber, though that is already available on the Play Store.) Of course, also included are bug fixes and security enhancements, which are part and parcel of Android 4.4.4.
The OTA update should be making its way to devices over the next few days; as usual, you can try and force it to show up by heading into the Settings » About phone » Software update menu, or just wait it out till the notification for the update pops up on your phone.