That depends on how you get the phone. You see, if the phone comes with 4.2 firmware, you'll be able to unroot and triangle away, with no issue. If you get the S4 and it comes with 4.3 (or if you update to 4.3 yourself) you wont be able to remove traces if you flash (for now) because 4.3 comes with a new bootloader that is locked and triangle away doesn't work. The second you try to flash something with 4.3, you'll trip the switch on the new bootloader and it will show you voided your warranty with no way to hide it. Also, once you get the 4.3 update you wont be able to odin back to 4.2. BUT If you stay on 4.2 and don't take the 4.3 update you'll be able to flash 4.3 ROMS (all roms dont include the bootloader) with no warranty issues. I'm curious to find out if the phone comes with 4.3 or 4.2. Let us know.
The unroot and remove traces for warranty process is the same with the S4 as other phones, I dont think that has changed. There may be some basic instructions in the
http://androidforums.com/metropcs-g...t/773372-unofficial-m919nuvuamf2-working.html thread.