If you flash a pre-Knox ROM you will blow the efuse and may (or may not) end up with warranty issues and won't be able to flash a stock ROM any more.
Some of the 4.2 ROMs, like the one my S4 came with, didn't have Knox, which is how I rooted my phone without blowing the efuse. You should be able to use one of the later ones after Samsung implemented the Knox functionality no problem though. I cannot remember which version this happened on but all the details are available in the root sub-forum.