My 4 month old has started to roll in his bassinet whilst sleeping. Is it ok for him to sleep on his tummy if he rolls there himself? Or should I roll him back to his starting position which would be me placing him on his back with his arms up? He looks comfortable, I’m just scared he will wake up and upset himself.

Most babies will be rolling over by 5-6 months…....some earlier, some later.
This is very normal as babies grow & become stronger many babies then prefer to sleep on their tummy.

It is important that you move your son from bassinet & cease wrapping if you have not already done that.
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.

Always place baby on their back to sleep.
Ensure cot mattress is firm, well-fitting & flat.
Ensure there is nothing loose or soft in the cot.
Preferable to use a sleeping bag now that has appropriate size neck & arm holes.

Red Nose has the following information now that your baby boy 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-over