My Samsung Galaxy S4 just notified me of a 21MB firmware update this afternoon, and as is often the case with these updates, it doesn't tell you what it does. First guess? Speed improvements to fix the lag some folks have noticed with their ATT model. Though ours hadn't lagged before, it feels a snappier now, and the camera (which did feel slow) is now more responsive. RAM usage is down to 700MB, which is 300 megs less than before the update. Owners on forums state that lag is much reduced. Good times.
The update brings a new kernel version and for those of you who have rooted, it will remove root. You can root this new version using motochopper, just as with the initial release.
If you don't want to wait for notification of the firmware update, you can manually check for the update by going to Settings, then selecting the rightmost tab (More), then select About Device, and finally select Software update. Again, this is for the AT&T version of the GS4.