My baby is 4 months old and has started to roll from his back to tummy but not from his tummy to his back. Initially we put him down on his back for sleeping but we find he Rolls onto his tummy very frequently during sleep. We already use a arms out sleeping bag. Should we be turning him on his back everytime we find him on his tummy or is it ok to leave him asleep on his tummy ? When we keep turning him back he keeps going on his tummy and this causes constant disruption to his sleep.

My baby is 4 months old and has started to roll from his back to tummy but not from his tummy to his back.
Initially we put him down on his back for sleeping but we find he Rolls onto his tummy very frequently during sleep. We already use a arms out sleeping bag.
Should we be turning him on his back
everytime we find him on his tummy or is it ok to leave him asleep on his tummy ?
When we keep turning him back he keeps going on his tummy and this causes constant disruption to his sleep.

Most babies will be rolling over by 4- 5 months. Some earlier, some later.
This is very normal as babies grow & become stronger.
Many babies then prefer to sleep on their side or tummy.
This is not something that needs to be stopped. It is important to make his new environment as safe as possible.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over

Is he in a cot now &/or are you still wrapping him?

Babies need their arms free & the space of a cot to roll well.
I think it would be important to move him to the cot now.
Is the cot mattress firm & flat?
How does he go with tummy time when awake? Is he holding her head well &/or reaching for toys?

Does he roll/wriggle around the cot (or floor) when awake?
Always still place him on his back for sleep.

Have a read of the Red Nose 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”
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over