Samsung has never been a manufacturer that wants to prevent its customers from loading custom ROMs onto its devices, but U.S. carriers like AT&T and Verizon have a polar opposite policy and often lock down their devices' bootloaders to keep everyone on the stock software. Now, owners of Verizon and AT&T's Galaxy S4 and AT&T's Galaxy S4 Active have a way to flash custom ROMs while keeping the stock ROM intact: meet SafeStrap recovery.
SafeStrap was originally devised for Motorola devices, which most often come with unlockable bootloaders. 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.
For now, SafeStrap for the Galaxy S4 and S4 Active is in an early stage, so you're recommended to heed caution while trying anything out (take full backups, etc.), or simply stay away till more stable versions are released, in case you're not too familiar with this stuff. The source link has all the details, so go ahead and give everything a read.
