Load custom ROMs without touching the original boot loader using Safestrap
Folks with a Galaxy S4 on AT&T or Verizon, as well as users with the Galaxy S4 Active have a new method of loading custom ROMs in the works — Safestrap. Originally born out of necessity for the Motorola Droid line — many of which were and still are locked down tightly — Safestrap allows you to load a custom ROM of your choosing after your phone is booted.
What happens is that a system process the runs at the end of the boot process gets hijacked by crafty Android hackers, and is pointed to a bit of custom code that loads a sort of virtual machine running a custom recovery, which can continue loading a ROM of the users choosing. The original ROM is still safe and sound, it's just not active. Of course this is a simplified version of events. For the more technically inclined, you'll want to have a look at the further discussion here.
Since AT&T and Verizon seem to have a need to get manufacturers — even ones who have been typically developer friendly like Samsung — to lock down their phones, seeing this come to the S4 and the S4 Active is really cool. The project is still in its infancy, so there will be bugs galore, but I have a feeling this one will grow into something very nice. Have a look at the link below for all the information you need to give it a try.
Source: XDA. Thanks, Matthew!