Originally Posted by jdelano View Post
If you had taken the 4.3 update with KNOX, you can not downgrade the bootloader to MD4 - this WILL brick your phone.
There are a number of threads highlighting this eventuality.

Anyway downgrade bootloader from 4.3 to 4.1?
[STOCK] 4.3 and up Discussion Thread (NOT CM or AOSP)

3-4 others in General. I've not found a "debrick" image for this phone as of yet.

There is this thread : [GUIDE]Unbrick a Hard Bricked SPRINT Galaxy S3 (Without JTAG) but I am not sure that would work for you.

I'm sorry this happened and I hope you can get your phone fixed.
I second that sorry as well. I hate to say it, but from what I have read and researched, when Knox prevents a bootloader change, it is the ultimate brick, as in not even a JTAG can fix it. Very crappy, and I think Samsung is going to take a small hit in the future from previous owners who move away from them the next time a device update is in store thanks to the garbage known as Knox that was forced on a lot of unsuspecting device owners.