Hi Lonelyprogger,
I think that you are talking about a key with 4 #'s, which would either be the key of E major or C# minor. The sharped notes are F# C# G# D# of course. You want to know what happens to C# if there is a natural beside the note that is on the C-line or C-space, given that in the key signature that C would be sharped.
Answer, every C in that measure (bar) will become C natural. The natural sign will only affect the notes in that measure and no other measure.
There are several different conventions currently in written music in regards to all the other C's. Most of the time (?) all of the C's in that bar, regardless of their position, would be affected by the natural sign. But in some editions, only the C's occuring on the same line or space are affected. In that case you would have to go by what sounds correct or makes sense in the music.