While no US carrier-branded variants of the Galaxy S4 have received an official update to Android 4.4/KitKat yet, Samsung has dropped the KitKat kernel source for the Sprint's model. While that by itself is really only of interest to developers, its implications will matter to a much larger audience. Generally, Samsung does not release kernel source for builds that aren't official. Historically, once source code is made available, official OTA updates follow in reasonably short order.
The new firmware build number is L720VPUFNAE. Currently, there is no official word from either Samsung or Sprint on when the accompanying OTA update will roll out, but if building kernels and hacking are your thing, go have fun.