Found a set of wheels of a c4s and wondered if they would fit my 99 c4?? Any ideas, Im pretty sure the offset is different

The fronts will be fine. The rears will be a problem. I think you can make them fit, but it may require spacers and/or rolling your fenders.

Is it because they are to wide?

I believe it's the offset and the width

it all depends if the 4s wheels needed spacers when on the 4s.

for example, the 18x11 sport techno wheels that came on 4s' were actually not a true wide body wheel, it came with 17mm spacers from the factory.

so if the wheels your looking at required a spacer for the 4s, they may fit your c4 without the spacer.