Last Updated: July 23rd, 2013
Some devices get all the ROMs. Of course, for the Google Play Edition of the Galaxy S4, that's kind of the point. Late last month an early version of Android 4.3 was leaked for the phone right around the time of its Play Store debut, and now an even newer build labeled "JWR66V" has popped up. According to XDA Developers forum poster Jahon, it was build on July the 11th, and now it's been re-packaged into a ROM file flashable in the custom recovery of your choice.
Last Updated: July 25th, 2013
Several hours ago, an Android 4.3 system dump was leaked for the Nexus 4. As it turns out, even though the bootloader and the radios weren't included, the system dump is totally bootable. I'm running it right now. If you want to try it out, it's easy to do so, but be prepared to have your bootloader unlocked and flash some zips via a custom recovery. If you don't know what any of this means, I suggest you get familiarized with Android flashing first.