Samsung is reported to have access to the source codes of the yet to be released Android 4.3 Jelly Bean which is being tested on the Galaxy S4 Google Edition. What is even more interesting is that the Android 4.3 ROMs for SGS4 GPE has even leaked online, which can be seen as a sort of a pseudo confirmation of Google actually readying the Android 4.3 update in the short term before the more feature laden Key Pie Lime is made available. Google though has never made any official mention of the Android 4.3 Jelly Bean version which is seen is a minor upgrade over the current version.
Also, the leaked ROM is being reported to be functional and has already been tried out on the Galaxy S4 version which comes with the Snapdragon 600 chip. Doing so makes sense as it is this version that bears a close resemblance with the Galaxy S4 Google Play Edition. However, it is strongly advised to not to try installing the Android 4.3 version on the Galaxy S4 devices that are based in the Exynos processor as the software in its present configuration is not compatible with that particular chip and might even brick the phone.
The Android version can be downloaded from the website sammobile.site and can be flashed with the help of a custom recovery like CWM or TWRP on GT-I9505. However, most of the changes that the Android 4.3 version stands for seems to be restricted to under the hood with there being almost no visually perceptible changes to be perceived.