Originally Posted by polobunny
You'll need JTAG service repair.
He may not if it is a soft brick. As long as when he says softbrick he means that it is not loading past the Samsung log or the ATT logo for example. I was on stock rooted 4.3 as well and tried flashing other roms as well and soft bricked my phone in the process.
I ended up with two choices, load only 4.3 (The STOCK MOD v4 install worked for me just fine - but I was trying to get out of 4.3 tbh) Tried several other roms and all would end in a soft brick.
Finally landed on a stock 4.1.2 UCMG2 rom http://forum.xda-developers.com/show....php?t=2388065
After clearing my internal SD and formatting it I flashed it and it worked (took a long time on the Samsung logo but worked!) the wifi is a little flaky at times but it always was on 4.1.2 for me anyways... the key I believe was that this rom did not have a bootloader and modem updated, which was required to properly downgrade after having had previously installed 4.3.
OP try it out and see how that works? Unfortunately I could not find a TAR of this, but luckily I was able to use a microSD adapter to move the needed files onto the SD card so that I could flash in TWRP.