This should probably be in the root section, but irregardless. If you are on ATT or Verizon and the newest firmware available, you cannot install custom recovery and therefor cannot install custom ROMs. But all the information you need is on xda-developers.com. That is where the vast majority of ROMs are, and they are categorized by phone, if you get a ROM for the galaxy s4 but not the correct carrier, that could permanently brick the phone. But with all custom ROMs, most apps work, like launchers etc. If you go with cyanogen, you lose the touchwiz framework and will lose the multiwindow and IR blaster functionality etc., unless they have found a way to include it. I personally dont know because I dont recommend getting a galaxy s4 if youre going to lose the touchwiz framework
now triangle away is something i have but have not used it yet. it should work and restore everything. If someone is really worried about having something happen and losing warranty, I generally dont recommend flashing ROMs because, if something does go wrong, youre out a lot of money. That said, I think its pretty safe to install ROMs, I've done it to every android device ive owned and never had a hard brick, but it is not a guarantee.