Stomach sleeping!

My baby just turned 4months old and has been waking himself up from rolling onto his stomach. Tonight when I went to check on him in his crib he had rolled onto his stomach and was asleep. He can easily roll onto his stomach but I have only seen him roll onto his back with a lot of effort and only once or twice. I'm so nervous to leave him on his stomach. Should I reposition him? Am I over reacting?