The leaked firmware update is for the European version of the Galaxy S4 and has been sent to SamMobile, from an inside source in the company. According to the testers, the leaked firmware is pretty stable, apart from a few bugs here and there.
Changes on the Android KitKat 4.4 version of TouchWiz seem minimal, Samsung has removed most of the light green icons and replaced them with white, to match the new aesthetic for Android 4.4.
There is a new camera icon on the lock-screen that the user can open, similar to iOS and stock Android. Samsung has also worked on the keyboard in landscape mode, since the original was not optimised and looked rather glitchy.
Early testers have said the firmware update does perform better than it did previously. Google had a lot of optimisation and made sure Android KitKat 4.4 could run on less power, two reason for the big boost in performance when smartphones get the update.
Other than that, there is nothing that has really changed, Samsung is keeping the same aesthetic in most places. This may just be a pre-build for the Galaxy S5 and Samsung may announce new features when they launch their next flagship.
The leakers did not say when the Galaxy S4 will be getting Android KitKat 4.4, we suspect it will be sometime in the next few months, probably a month or two after the Galaxy S5 has been announced and new features are available.