If youíre looking for a Samsung Galaxy S 4 that you can easily flash custom firmware to, you might want to steer clear of AT&Tís version. Steve Kondik has confirmed that device has a locked bootloader. For the average customer this might not be a huge deal but thereís a huge segment of users that want to root and flash third party ROMís to their device and thatís not going to be an easy feat with this device. Iím pretty sure its just a matter of time before this thing is cracked but to avoid a possible bricked device, T-Mobile and Sprint seems to left their bootloader unlocked out the gate so either of those two carriers would be a better choice.
AT&T Samsung Galaxy S 4 has a locked bootloader