Not that Android 4.3 is anything official yet, we are still awaiting the official announcement from Google, but it's obvious that it's sooner or later coming. For Samsung Galaxy S4 user it's coming sooner because SamMobile has obtained the first Android 4.3 test build for the Google Play Edition of the Galaxy S4 (GT-I9505G). As they wrote, for sure they wanted to test it out but figured out it requires the Google variant of the Galaxy S4. Therefore they ported it to their standard Snapdragon-powered Galaxy S4 (GT-I9505) and got some outstanding results. SamMobile is saying that the Android 4.3 port is fully functional on the standard Snapdragon-powered Galaxy S4 and that there's not even a single thing which isn't working.
Furthermore they continued that they haven't made any changes to the port, so it's 100% original with zero changes, as developed by Google. Nevertheless, it was also noted that the current Android 4.3 version, SamMobile got its hands on, does not have all the features yet which are still to be implemented. Therefore it'S safe to say Android 4.3 seems to be not feature complete but it can't take too long anymore. An official release is expected for fall this year.
Cheers ~ Arne