Will 4th gen stock wheels(the 16" 5 spoke style) fit a 3rd gen? 1991 camaro to be specific.

 

The bolt circle is the same, but the backspace may be too large. Some 3rd Gens came stock with spacers because of the narrow rear axle assembly (3" narrower than the 4th Gen). The 4th Gen stock wheel has a 6-5/8" backspace. Measure in that distance from the hub mounting surface, and you will see how deep the wheels will sit in the wheel wells.

 

I run 17" 10 spoke rims on my car off a 2k1 SS. You'll need 2" spacers all around for any 4th gen rim. Or you can do the fun thing and swap in a 4th gen rear and you'll only need spacers for the front.