Samsung, in their infinite wisdom, have released the latest Galaxy S4 stock firmwares, MGG
and all future firmwares I am given to understand, with locked bootloaders
Their reasoning, I believe, behind this is plug security exploits such as the Master Key
This obviously has implications for flashing custom ROM's which require an unlocked bootloader.
Even as I type, dev's are working on unlocking the bootloader, notably chainfire
, and hopefully it will not be too long before this happens.
Until such time as a stable workaround is available the best advice is to avoid flashing any stock firmware or ROM's with a locked bootloader. DjeMBeY, for instance, has withdrawn his stock, rooted, MGG firmware because of this.
I am sure that someone like Hawker
can give a better, more detailed, updated, explanation regarding the situation... providing we can sober him up!