Its disappeared already, so presumably nothing torn or broken.
Its probably just a sprain or suchlike, butnp it keeps recurring. At 4 hes far from ancient so im hoping a week of short then gradually longer walks will fix it before slowly reintroducing his ball.
He absolutely loves his ball so would be devastated if he couldn't have it any more, but ifmits a persistent issue I would do that. Labs can be prone to joint problems and the like so if it proves to be a persistent issue id do that, grudgingly, to reduce the risk of more serious problems in later life. But I'll see how we get on going gently again and building back up.