Hi, my son has just turned 4 months old. He can roll one way, and keeps trying to settle on tummy at night time. He is too big for bassinet now, I want to put him into his cot but some articles say he should stay in our room until at least 6 months old? The bassinet is now not really safe for him?

What you are describing is normal development.
Most babies will be rolling over by 5 months…....some earlier, some later.
This is very normal as babies grow & become stronger many babies then prefer to sleep on their tummy, & often don’t change from that position.
While your son is younger, it is not uncommon for a 4 month old to roll.

Red Nose actually recommends to have baby’s arms out of any wraps at 3 months in preparation for rolling.
Modify the wrap to meet the baby’s developmental changes, e.g. arms free once ‘startle’ reflex begins to disappear which is usually around 3 months
https://rednose.org.au/article/wrapping-babies

Are you still wrapping your son? If so, it is important to cease wrapping (or using a bag that keeps arms in) to ensure arms are free now that he is rolling.

It is also important that he is moved to his cot, now that he is rolling.

This is not something that needs to be stopped, but it is important to make the environment as safe as possible for this to occur.

Always place baby on their back to sleep.
Sleep baby in cot now, not bassinet.
Ensure cot mattress is firm, well-fitting & flat.
Ensure there is nothing loose or soft in the cot.
Use a sleeping bag now that has appropriate size neck & arm holes…....... no wrap or swaddle.

Red Nose has the following information now that your baby is starting to roll.

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.
By this stage it is not necessary to wake during the night to turn baby over to the back position

Do not use any devices designed to keep baby in a particular sleep position.
https://rednose.com.au/article/what-do-we-do-now-that-a-baby-has-started-to-roll-over
https://rednose.org.au/article/is-it-safe-to-use-products-designed-to-keep-baby-in-a-particular-position
The sleep environment takes priority over room sharing.
Parents are not expected to observe baby constantly.
If baby is sleeping in a separate room check baby regularly to ensure that the baby remains on the back and the head and face remain uncovered (as baby grows beyond 5-6 months they will move around the cot and roll over; settle baby to sleep on their back but let them find the sleep position they feel most comfortable in.
A safe cot and safe sleep environment is still necessary for older babies).
https://rednose.org.au/article/roomsharing