Hi, my son (4 months) just rolled over today, back to tummy. He slept at night in a bassinet in our room, day sleeps in his cot in his room. I'm not sure what to do now, I know the bassinet isn't safe now, but worry the cot isn't either as he can't roll tummy to back yet. He also hates tummy time and will place his face down on the floor, I'm terrified if I leave him in the cot at night that he could do that.

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

He needs to be in his cot now & unswaddled….. arms free.
Babies need their arms free & the space of a cot to roll well.
You really needs to persist with tummy time as this helps to increase his upper body strength to assist with rolling.
Have a look at info on tummy time info.
https://rednose.org.au/article/the-importance-of-play
https://rednose.org.au/downloads/RN0407_22_Tummy-Time.pdf

Helping him play with toys can help babies stay on their tummy for longer.
You can always do shorter periods more frequently, rather than longer periods.

Remember, still always place him on his back for sleep.
Have you been rolling him back when you see him sleeping on his tummy?

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