On 4-Stars, go with film, Monokote, Ultracote, etc.

I think Sig states not to use fabric on the 4-Stars. Here's why.

Film is stronger for RC than fabric. I know what you're thinking, but read on. The fabric will stretch and allow the underlying framework to break. Film may tear easier, but it won't stretch like fabric and it does make your framework stronger.

Sig's planes, especially the 4-Stars, and I have a 60 bashed into a Kawasaki Tony, have unsheeted wings with multi-stringers. The wings can be fairly "twisty." This can cause stringer failure. I think they specify Monokote to stiffen and strengthen the wing.