At this point of time, the only thing we know about Android 4.3 is that it does not exist outside of the Google headquarters. The new camera app, which everyone assumed is going to come in Android 4.3, turned out to be a new camera app on the Google Play edition handsets.
However, today, the folks over at SamMobile have managed to get their hands on the first Android 4.3 firmware. The firmware was initially meant for the Galaxy S4 Google Play edition handset, but has been ported over to the normal I9505 Galaxy S4 by them.
The Android 4.3 based firmware will work on the I9505 variant of the Galaxy S4, and you can install it, provided you have root access and a custom recovery installed.
Visibly, there are no noticable changes that Android 4.3 Jelly Bean brings. However, under the hood, it includes support for Bluetooth LE, lots of new API, better GPU rendering for apps and other.