Samsung has released the source code for the Sprint Galaxy S4′s KitKat update. An update’s source code usually isn’t released until after the update has begun rolling out, but Samsung decided to release the source code first. This means that the final build cant be far behind.
The update should start going out at any time, but until it does we're sure developers will do their magic. When the update does begin pushing, as of now Sprint will be the first in the United States. There is no official word so far from Samsung or Sprint on the matter.