is your baby sleeping on its tummy?

Nathaly
My 4 month old can flip from back to belly and seems to love flipping to his belly when it's time to go to sleep. I try to flip him on his back but eventually he goes back to belly and I don't want to wake him up. Is it ok for him to be sleeping on his belly at this  point? They say back is best all the time so I'm not sure what to do.