My friend @paulove have just flashed the new XXUEMK4 from here:
His bootloader was updated to MK4 but with a little "gift" from samsung:
Knox Void Warranty 0
When he tried to flash recovery and it changed to Knox Void Warranty 1
Please everyone, read this to understand everything you need about Knox Void Warranty 1/0:
DO NOT FLASH THE NEW BOOTLOADER - Unless you know what you are doing!
MJ5 Bootloader is safe to downgrade and the wifi works on this bootloader.
XXUEMK4 Bootloader is not safe!!
My friend says that he can not downgrade, because after he tried he got "Kies Emergency Mode" and his odin failed.
After he installed MK4 back everything worked again.
DO NOT TRY TO DOWNGRADE IF YOU HAVE THE NEW BOOTLOADER!