Hi, my baby boy turns 4 months next week and he has started rolling onto his side during his sleep, and has now started to roll onto his belly. His head is turned out so he can breath. Is it safe to let him sleep in this position if he puts himself there or do i need to roll him back into his back. He can only roll from back to front at this stage and During the night and day naps he sleeps in his own cot in a separate room as we dont have space to put his cot in our room, which is only about 1.5/2m away from our room.

It is not uncommon for babies to be rolling by 4 mths.
Most babies will be rolling over by 5 months. This is very normal as babies grow & become stronger.

Many babies then prefer to sleep on their tummy.

It is important that you cease wrapping if you have not already.  Your son needs “arms free” to roll safely.
Also ensure that there is nothing soft or loose in his cot.
This is not something that needs to be stopped, but it is important to make the environment as safe as possible for this to occur.

Red Nose has the following information now that your baby is starting to roll.

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.com.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-ove
Red Nose also has the following information about Room Sharing.

Safety of the baby’s sleep environment is a priority over sharing the same room as the baby…
Parents are not expected to observe baby constantly.
If baby is sleeping in a separate room check baby regularly to ensure that the baby remains on the back and the head and face remain uncovered
(as baby grows beyond 5-6 months they will move around the cot and roll over; settle baby to sleep on their back but let them find the sleep position they feel most comfortable in.
A safe cot and safe sleep environment is still necessary for older babies).
https://rednose.org.au/article/roomsharing