Hello My almost 4 month old has just started to roll on to his stomach not only while he’s awake but now also during his sleep. I’m constantly putting him back on his back but it’s just a loosing battle. He has a lot of tummy time during the day and has quite a strong neck the only issue is he can’t roll himself back once his on his stomach. Sometimes I find him with his face completely face down. He has never slept swaddled as he doesn’t like it and has his arms and legs completely free. What can I do to stop him rolling on to his stomach until he can roll Back over ?

Most babies will be rolling over by 4- 5 months…...some earlier, some later.                             
This is normal as babies grow & become stronger.
Many babies then prefer to sleep on their tummy & some seem to face plant.
This is not something that needs to be stopped. It is important to make his environment as safe as possible.

Ensure that the cot mattress is firm & flat & there is nothing soft or loose in his cot, especially as he is face planting at times.
You can try to gently turn his face if you see him with face down, but often, like rolling to tummy, babies will just go back to their position of comfort.
As you have found, there is not much to be achieved by trying to roll him back from his tummy.
If he is still in a bassinet, he needs to be moved to his cot.
Babies need arms free & the space of a cot to roll well.
Always continue to place baby on back to sleep.

Red Nose has the following general information now that he is rolling.

Give baby extra tummy time to play when awake and supervised as this helps baby to develop stronger neck and upper body muscles which in turn enables them to roll back over.
Make sure that baby is on a firm and well-fitting mattress that is flat (not tilted or elevated).
Make sure that baby’s face and head remains uncovered (do not use lambswool, doonas, pillows, cot bumpers or soft toys)

As babies grow and develop they become very active and learn to roll around the cot.
At this time, continue to put them on the back at the start of sleep time, but let them find their own position of comfort.
By this stage it is not necessary to wake during the night to turn baby over to the back position

Do not use any devices designed to keep baby in a particular sleep position.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over