New features, but with very few noticeable UI changes
Right on the heels of the Galaxy Note 3 launching with Android 4.3 on board, a prerelease build of Android 4.3 for the Galaxy S4 has leaked out. The folks over at SamMobile have released a test build of the upcoming firmware for willing users to take a look at, and they claim that it is relatively stable. The biggest updates included here are new software features that we find on the Galaxy Note 3, along with lots of under-the-hood fixes and improvements.
The build is made for the unlocked European model (GT-I9505) of the handset, and the site claims that it is ready to be loaded on your own phone for testing if you are willing to deal with some issues. As with any other prerelease or test firmware there's always a chance that you could run into issues, so be sure to do your research and flash with caution.
More: Samsung Galaxy S4 Forums