Found a full box of "Supreme" #4 12 gauge shells in my uncle's old room. The box was opened, but no shells were used, it's from the early 90's, possibly late 80's, according to him. Are they valuable in any way, or should I just blast them at skeet?

Supremes were Canadian I believe.

They won't be worth a hell of a lot, VERY few (basically none) plastic shells are worth more than modern equivalents. If anything, the box will be worth more than the shells, but probably not.

However, I don't believe there is ANY gun club in existence that would appreciate you using them for clay targets, unless you're just talking about clay pigeons off a hand trap in the back yard.

Yeah, i do mean in the back yard, but just out curiosity why would a club take issue with them? Is it the shot size?

Yes.