There are major upgrades, like the Android 4.3 update that everyone waited so long for, and there are minor ones, like the one Sprint is pushing out to the Galaxy S4 right now. Like, extremely minor – the MK2 update resolves sleep mode issues that some users were apparently facing, but doesn't do anything other than that. However, considering poor battery life was one of the issues for many folks with the Android 4.3 update that rolled out a couple of weeks back, it's perhaps an important update despite the rather short changelog.
The MK2 update has been rolling out since November 30th, so you should already have received a notification for it, though you can do a manual check in the Settings » About phone » Software updates menu if you haven't. Do let us know if you notice anything new or different once you've updated your S4.
