My 4.5 month is able to roll from back to tummy but has not been able to learn to roll from tummy to back. She sleeps with her arms free and in a cot which is within our room. She has recently shown a liking to sleeping on her side and rolls over to her side despite being placed on her back. If she does go fully onto her tummy at night, she will cry and we will reset her. This is quite disruptive and impacts her and our sleep. Do we need to roll her on to her back or can we leave her on the side.

Most babies will be rolling over by 4- 5 months…...some earlier, some later.                             
This is normal as babies grow & become stronger.
Many babies then prefer to sleep on their tummy or side.
This is not something that needs to be stopped. It is important to make his environment as safe as possible.

Glad to see you are aware of what to do when baby is rolling & that you have stopped swaddling & moved her from bassinet
She needs her arms free & the space of a cot to roll well.

Ensure her mattress is firm & flat & that there is nothingloose or soft in the cot.
How does she go with tummy time when awake?
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 . 
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.

Do not use any devices designed to keep baby in a particular sleep position.
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over

I’m sure she will soon be seen to 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.