Samsung Galaxy S4 and S4 Active users on AT&T and Verizon networks now have the possibilities to flash their phones with custom ROMs. This is now achieved using SafeStrap recovery procedure.
Samsung is not one of those manufacturers who wants to lock their bootloader and tries to stop their users loading custom ROMs on their devices, but operators like AT&T and Verizon have strict policies against rooting their devices and using custom ROMs like CyanogenMod. But gone are those restrictions. Now you can use ROMs of your own choice.
SafeStrap recovery was originally developed for Motorola devices where the bootloader were often locked. What SafeStrap does is create a virtual environment of sorts to run your custom ROM through the user storage, by hijacking the device’s boot process, while keeping the stock ROM safe and sound on the system partition. As it runs custom ROMs virtually and through the user storage, you can even have multiple custom ROMs installed that are assigned to different “ROM slots,” something which isn’t possible using recoveries like ClockworkMod and standard ROM flashing procedures.
To get to know more about the whole procedure checkout XDA developers forum for more information.