My baby is almost 4 months old and has been rolling for about 3 weeks now, when we put her down to sleep we put her on her back but she straight away rolls onto her side or sometimes her face, we go in and reposition but same thing happens. What do I do? Is this safe because she can roll now ?

Most babies will be rolling over by 4- 5 months. 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

Have you ceased swaddling her?
Babies need their arms free & the space of a cot to roll well.
How does she go with tummy time when awake? Is she holding her head well &/or reaching for toys?
Is the cot mattress firm & flat?

Always still place her on his 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”
https://rednose.org.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over