This is for all of the Galaxy S4 users out there that are looking for the factory firmware for their Galaxy S4. I have this phone and had a tough time finding the OTA 4.3 firmware files. After found them on the XDA Developer Forum I decided to make them available here in case others were having the same problem.
The first file is the largest and includes the factory OTA 4.3 firmware I537UCUBML2 to include a restore image that is also installed.
I can only assume that the second and third files include updates, if anybody knows exactly what these files do please email me at Doug@sightsbysight.com
This fourth file appears to be some sort of AT&T update. After flashing this file the flash screen on boot up says “CUSTOM” with the open lock?
I flashed these files using ODIN V3.09 with the options set to Auto Reboot and F.Reset Time checked and flashed via AP , BL, CP, CSC.
All I can say is that these files worked for me perfectly. At first I flashed just the first file (AP) but then I got to wondering about the others and ended up flashing all 4 of them without any problem, Just the same I would still caution you to use at your own risk. I am in no way endorsing the outcome on your phone.