4 year old behaving like a 2 year old

[Fun&care]

I think you should say something. You don't have to say AUTISM, but you can mention that in comparison with the other kids his age that you have noticed he seems a bit behind in certain areas. Ask if they have noticed and/or if they have brought it up with their doctor. I think not saying anything is the worst thing you could do, qualified or not. You don't need to make a diagnosis, just point out your observations, after that the ball is in their court and you can have a clear conscience about it.