Yes, I agree, 9 pm is late for 4 yo and I'd say older. But if he is happy and not OT during the day, and gets up easily in the morning, then maybe it is. I know some mums whose kids are 4 and wake up at 5am, bed after 8. I'd be trying to "fix"that! Another thought - is it related to lighter nights - did it happen during the winter too.
Incidentally, I've just come back from a school trip. Nearly all the kids in the class slept in the bus on the way back. My DS didn't - talked ALL the way back (1.5 hour long). Is your DS like that? I think it is just his character, to not miss a thing. So, I'm happy as long as he is quiet once he goes to bed. He may not go to sleep, but he is resting (occasionally I hear him moving). I think it is just him and he takes a while to wind down. More to the point - it means I'm off duty at 8.30, and I really need that!