Software build I9505XXUEMJ3 is the latest version of Android 4.3 to leak for the Samsung Galaxy S4 and is said to be pretty much the final version of the update for that particular phone. Those who have tested the new build say that there are no major changes compared to the previous one, just some minor tweaks here and there.
The update is for the unlocked European version of the Samsung Galaxy S4 (GT-i9505) and while you can visit the sourcelink and download the update, remember that it is not finished. The usual caveats apply here about possibly bricking your phone and losing your warranty. With the official rollout expected to begin soon, why not just wait?
source: SamMobile via AndroidAuthority