Bub has been rolling from back to tummy for a month, he's 4 months old but he still can't roll from tummy to back. At night I've been waking to him sleeping on his tummy so I've been trying to move him onto his back but he wakes frequently wanting to roll again onto his tummy. He has slept with his head to the side, but I'm not sure if I should be worried that he could put his face into the mattress. Should continue to wake him by moving him back onto his back to 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 tummy &/or side.

**Now that your baby is rolling, he needs to be in his cot for all sleeps & not wrapped or swaddled, if not already in place.
Baby needs arms free & space of the cot to roll safely.
Increase tummy time when awake to help build upper body strength which is important when rolling.

It is important to make the environment as safe as possible to allow baby to roll freely.
Ensure that the cot mattress is firm & flat & there is nothing soft or loose in his cot.

As you have found, there is not much to be achieved by trying to roll him back from his tummy.
**Always continue to place baby on back to sleep.
Preferable to use a sleeping bag when rolling that has appropriate size neck & arm holes.

Red Nose has the following information when baby 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.
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