When a dog has Perthes it holds the leg up almost constantly, the skipping is just an occasional thing.
Perthes disease usually comes out in them when they're young though, I'm lead to believe around 6-7 months old, so it may not be that if your dog is now 4 years old.
If it is Perthes disease then don't breed off the dog, because its a genetic disease, so there stands a chance his offspring could inherit it.
Best of luck with the dog mate.