Looking through a songbook yesterday, I noticed a tune (Gaudete) in the key of 4 sharps - nothing wrong with that, except that the sharp symbols were on F, C, G , and C (instead of D)

What would a musician be expected to do if sight-reading? Just see the four sharps in the key signature and assume the key, or play the sharps as printed?

Further thought: If the former, where does this leave the suggestions made for modal key signatures, which may differ from the usual major/minor signatures?