Which version of KitKat do you have? If you have the leak (MLG) WITHOUT the NC2 bootloader, then you can go back using the MJ5 restore. If you have the official AT&T KitKat (NC2) then you are stuck.
Boot into download mode to see which bootloader you have. To do this you need to turn off your phone and hold down the home/power/volume down buttons all at the same time until the phone boots up. After accepting the warning about custom software, look for a list of letter/number combinations (I think it's the third line from the bottom?). If yours says A2 then you can go back to 4.3. If it says A3 then you're stuck with no way out.