I am working on making Nicks skeg with the help of his plans and put one layer of cloth on both sides 4 oz. It looks a little skinny and wondering if I should put another layer for strength?

The only place I might consider added reinforcement is right up at the pivot. The hook may end up with some short grain that is probably the weakest link. A an extra layer of glass on either side before putting in the spacer at the pivot may have some benefit. But for the rest of the fin, I really don't think it needs anything more than one layer. The wood and glass composite is plenty strong for the amount of force it will receive. But, if it makes you feel better, go for it. No harm.