Is there any way to restore my rooted S4 (I337 on 4.3) back to stock unrooted?
It depends on how you got to where you are now but quite possibly not.
If you blew your eFuse rooting your phone, you won't be able to. If you managed to root and upgrade before you ever installed a stock ROM with Knox security (as I was fortunate to be able to) then you shouldn't have any problem flashing a stock ROM.
Ironass knows more about this than I do and has posted explanations about the implications of Knox Security in the rooting sub-forum.
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