4 months old nap on tummy during playtime

Hi mummies, if my 4 months old fell asleep (short day naps) during tummy time, do i let him be or really need to change his position to sleeping on his back? he lay his head sideways when he fell asleep. And yes his neck is strong, he knows how to flip from back to tummy, and he would lift his head when he wake up. Supervised nap infront of our eyes.