Hi There, My daughter is 4 months old and is able to roll from her back to her tummy. She is only able to do this in one direction and cannot roll from her tummy to back. We place her into her cot on her back but she will now usually roll onto her tummy with her arms sometimes under her and usually with her face down (nose and mouth pressed into the mattress). When we roll her onto her back she usually cries and ends up back on her face.. I'm concerned she won't be able to breathe and not sure what to do or how to keep her on her back during the night.

Most babies will be rolling over by 4- 5 months. This is very normal as babies grow & become stronger.
Many babies then will roll & 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

Good that she in a cot now, not a bassinet.
Are you still wrapping/swaddling her?
She needs the space of a cot & arms free to roll well.

Ensure her mattress is firm & flat & that there is nothing loose or soft in the cot.
How does she go with tummy time when awake?
Is she holding her head well, & reaching for toys?
Now that she is learning to roll, increase her tummy time when awake to help strengthen her upper body muscles.
Always continue to place baby on back for sleep.

Have a read of the Red Nose information now that she is rolling . 
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over

You are not expected to keep waking at night to roll her back, especially as she rolls straight back.
If you are awake & see her sleeping face down, you could try gently turning her face to the side, but some babies seem to want to sleep face down. Ensure her mattress is very firm, so her face & nose is not completely enveloped

There is no product that is recommended for babies to keep them on their back. Rolling is a normal development stage.
It is all about making their environment safe
I’m sure she will soon be seen to regularly roll both ways - some babies take a little while to want to roll back from their tummy, but she probably changes her position during the night at some stage.