Each baby has different pacing so you don't need to be too worried. The app has a list of red flags you should watch out for. At 4 months about 50% of babies can roll over so it's normal of they can't yet. Just do plenty of tummy time! Mine only turned at 5 months plus btw
So long they grow then is fine. Last time, I did try to roll in front of him to teach him! Lol. My baby rolled at about 5months old...