My 4 month old has started rolling and although we always put him to sleep on his back, tonight he turned himself to sleep on his tummy. The below article suggests that it’s okay to no disturb babies who turn themselves to sleep on their tummy over the age of 6 months, given this is the typical age for rolling. My question is, given my LO has started turning himself, should he turn on his tummy while sleeping, is it safe for me to leave him like that? https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over

What you are describing is normal development.
Most babies will be rolling over by 4 - 6 months…. some earlier
This is very normal as babies grow & become stronger many babies then prefer to sleep on their tummy, & often don’t change from that position.
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.
Transfer to cot if not already.
Ensure cot mattress is firm, well-fitting & flat.
Ensure there is nothing loose or soft in the cot.
Use a sleeping bag now that has appropriate size neck & arm holes if appropriate….. cease wrapping if you have not already done so.

It is essential to discontinue wrapping as soon as baby starts showing signs that they can begin to roll..
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over

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 lamb’s wool, 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
https://rednose.org.au/article/is-it-safe-to-use-products-designed-to-keep-baby-in-a-particular-position

If you are concerned about him rolling back, it may be helpful to increase his tummy time while awake.