My 4 month old has been rolling back to tummy since 3 months but can’t roll tummy to back. He rolls over in his sleep and seems to be comfortable on his belly sleeping. I always place him on his back when first going to sleep. Is it safe for him to sleep on his belly if he rolls over, even though he can’t roll back yet or should I be turning him back over onto his back?

Is he in a bassinet or cot?
Are you still wrapping/swaddling him?
How does he go with tummy time when awake?
Have you seen him roll from tummy to back when awake?
What does he do if you roll him back at night when you see him sleeping on his tummy?

This is the general information about rolling
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 the “new” environment as safe as possible.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over

Continue to always place him on his back for sleep.