We covered the 4EXT custom recovery when it was first released for the HTC EVO 3D, and now we're talking about it again because the updated version, v18.104.22.168 RC1 Test, brings with it some nifty new features.
Inspired by TeamWin's Dumlock, 4EXT will detect any pending changes to the kernel/boot images and flash them in the right mode. If you're S-ON, this means rebooting and flashing; if you're S-OFF, this means just flashing from recovery.
Unlike Dumlock, 4EXT does not require flashing the boot image twice, but an extra eight-second reboot to flash is involved if you've changed the boot.img and are S-ON. Those who are S-OFF can save these eight seconds, and should current S-ON folk become S-OFF at a later date with a method that doesn't wipe out your recovery, they can simply toggle the recovery's method of flashing via Recovery Control or editing the 4ext.prop file.
As this is a release candidate and labeled as a test, use at your own risk. I flashed a new kernel, did a Nandroid, basically ran 4ext through its paces and it never stumbled, but if you're skittish, wait until it's declared stable or bricked EVOs are piled high and the lamentations of the unfortunate victims are heard.