What problems could I encounter putting c5 wheels on my c4?

Possible none, I've seen a few C4's with C5 rims. Ask around some more. I know some of those magazines(MIdAmerica, Ecklers) sells C5 and C6 rims for the C4.

Most GM Vehicles have the same bolt pattern. The issues are always with the offset. If ya don't go deep dish, you'll be okay. I went the camaro wheel route, since everyone was chasing C5 and C6 wheels. I have the original 16in. Camaro wheels are all 16in except the 2011 ones. good Luck

Late modle C4's usually don't have a problem, its the early ones that need spacers