I have a 3 and a half month old who has started rolling from back to side and sometimes to her tummy when she sleeps. I've stopped swaddling and have her in a sleeping bag with arms free and she is out of the bassinet and sleeping in a cot with nothing else in the space. We also do tummy time and practice rolling from front to back but she's not quite there yet. I've been getting up every hour at night to check on her as I'm really worried, and move her onto her back if she's on her tummy or side. I've noticed that she often just moves back into these positions and can get upset if I keep moving her onto her back. What's the best thing to do as I know she is still quite young and in the 2-4month high risk period for SIDS?

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 her new environment as safe as possible.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over

You are doing all the things we would recommend - arms free, space of a cot, firm, flat mattress & tummy time.
Remember, still always place her on his back for sleep.

Try increasing her tummy time when awake - more frequently if she is not tolerating long periods on her tummy.
Is she holding her head up &/or reaching for toys?
Increased tummy time when awake helps to build upper body strength to assist with rolling both ways.
Continue with nothing loose, soft or padded in her cot.

If you wake & see her on her tummy, try gently rolling her back, but if she keeps resisting & rolling straight back to her tummy & becoming unsettled, then this is not really achieving anything.
It is not sustainable for you to sit up all night watching her.