Hello, our 4 week old baby sleeps in an arms up sleeping bag (Love to Dream swaddle up). During tummy time today he rolled himself from his tummy to his back, but he hasn’t shown any signs of rolling from back to tummy. Do we need to move him to a transitional sleeping bag with the arms out already or can we wait until we see him roll from back to tummy?

Sometimes baby can accidentally (or intentionally) roll from tummy to back at an early age.
He may be getting close to rolling. You could try him with his arms out of the swaddle, if you think he might start rolling during sleep.
Certainly, if he were to roll from back to tummy, especially when sleeping it would be important to following the guidelines for when a baby is rolling:
Cease wrapping
Move from bassinet to cot.
Continue to place baby on back for sleep.

Red Nose has the following information about swaddle suits:
Wrapping and swaddle products.
It is important to be aware that there is very limited evidence to support the use of specific products as devices to promote infant settling, back sleeping position and no evidence to support these products as a strategy to reduce the risk of infant death. Parents are advised to follow the principles of safe wrapping if they choose to use infant wrapping as an infant care strategy.
The material of the wrap or swaddle should not cover the face or head, particular if baby sleeps with arms in different positions. If the item is too big for the baby, some zipped swaddle suits that enclose baby’s hands, have been shown to allow material to cover baby’s face and nose when baby raises their hands above their head during sleep. All sleeping attire designed to cover the baby’s shoulders should have separate neck and arm holes or should ensure that they do not allow the face covering if the baby was to move their arms in different positions.
Any product that is used as clothing on the baby or in the baby’s sleep environment should not restrict the movement of a baby who is able to roll. Wrapping should be discontinued as soon as the baby shows the first signs of being able to roll. Positioning aids that restrict movement of the baby are not recommended and have been associated with infant deaths.
https://rednose.org.au/article/wrapping-babies