Not all smartphones are created equal especially when it comes to being developer and modder friendly, and Samsung's latest smartphones, it's Galaxy S 4 flagship and hardy Galaxy S 4 Active, are definitely no exception. But now owners who purchased said devices under AT&T and Verizon have a safer resort with Safestrap, allowing them to boot custom ROMs without removing the stock image.
The way Safestrap works is almost ingenious. It hijacks the regular Android boot process and points it to a virtual machine that runs a customized recovery software which then allows loading a custom ROM normally. This allows users to run such ROMs without having to override the default image provided by the manufacturer and carrier.
Installing Safestrap on supported devices is as easy as installing an APK, agreeing to a disclaimer not try to sue or hunt down the developer, rebooting to the custom recovery and following the usual process of installing custom ROMs. Safestrap makes use of virtual slots that allow users to have up to 4 custom ROMs. As of the latest version, Safestrap is based on the popular TWRP custom recovery but heavily modifies it so a pure TWRP would not work. It is recommended not to use any other bootloader or custom recovery with Safestrap.
While Safestrap is indeed advertised to be safe, at least compared to other more destructive and permanent methods of installing custom ROMs, the current version is still a work in progress. If you're an owner of one of the supported smartphones and would like to try it out, please do be careful and make sure properly back up.