Our 4 month old has started sleeping on her side or stomach in her cot. She's able to roll back to tummy but can't roll tummy to back yet. If we notice her on her stomach/side when we're awake we will try to gently roll her to her back but she often immediately goes back to her side/tummy. If we continue adjusting her she generally wakes upset. If she has good neck strength, are we ok to leave her on her side/tummy, even if she can't roll tummy to back yet? She sleeps arms free and we're trying to increase tummy time to build strength.

Red Nose recommendations are to “always place baby on back for sleep” not baby must always sleep on their back.
https://rednose.org.au/article/red-nose-safe-sleeping-recommendations

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 

Your baby does need to be in a cot now (as you have done) & also swaddling/wrapping ceased.
Babies been the space of a cot & arms free to roll well.
Ensure mattress is firm & flat & nothing soft or loose in his cot.
Continue to always place your baby on back for sleep.

How does she go with tummy time when awake?
Is she reaching out for toys as well as having good neck control?
Good to see you are increasing her tummy time. If she does not tolerate longer periods of tummy time try to increase the amount of time per day she has tummy time.
Are you still Room Sharing?

Until you are comfortable that she is able to roll both ways, then we would encourage you to gently roll her back, particularly when you see her on her tummy, but you are not expected to sit up all night watching her.
The safe environment continues to remains important.
However, if she continues to roll straight back to her tummy, then it may not be achieving anything safer.
Still always place her on her back for sleep, even when she is rolling well both ways.