At this point, don't make it a battle nor punish/scold him for it.
Keep in mind, that developmentally and per cognition, their imaginations ARE changing. And from even 2 years old, a child begins to have nightmares and/or "fears" of general things. Which to "adults" does not make sense.
Keep in mind, that we cannot turn "off" their imaginations or nightmares.
Kids this age don't even have, fully developed emotions, nor deductive or inductive reasoning.
I went through that too, when I was young.
My kids did, too.
My parents, let me sleep with them. They made no big deal about it. Because, they knew I would grow out of it. And I did. I too, would get scared at night etc.
But, how they treated me about it, is one of my fondest memories of them, as a child. How a child goes to bed or not, will be remembered.
My sibling on the other hand, was a cold fish and would make fun of me. I also, remember that.
For my kids: We had a floor futon in our room. My kids could sleep there if need be. We explained that to them. They went there on their own, and would sleep. They didn't wake us, and they fell asleep. They GOT sleep. That way.
They grow out of it.
Keep that in mind.
The main thing is, that he gets sleep.
All of this is developmental.
And we cannot turn that off.
Sure we can rationalize it with the child. But, that does not mean, it makes it "go away" for them.
Just let him, sleep in your room, but on a floor mattress or blanket or something. Explain that to him.
Plus, you have cut out naps.
At that age, my son was STILL napping, everyday, routinely. HE needed it. And even in Kindergarten, the kids napped. It was full day Kindergarten.
My son, was a NAPPER. And NEEDED it. At that age and older. Even at 5 years old, he napped. It was what he, needed. And he knew that as well. I went by my kids' cues. For naps/sleep.
My daughter on the other hand, did not need naps as much as he did.
NOW, keep in mind, also: that once your son DOES start Kindergarten... school is hard "work" for them. At the end of the day, and when you pick him up, they will be tired. ALL day at school, they have to be spot on. Listen to directions, doing things, etc. And this, tires them out. They are concentrating on SO much, at school. So after school, a child NEEDS TO deflate... and get out their yah-yahs. Even nap if they need it. They are little, and have little thresholds for things... and cannot keep up being "perfect" ALL day. Once at home after school, it is COMMON that a kid NEEDS to unwind. Deflate. And not do anything, right away.
Remember that.
You do not have to take away his naps, just because he is going to Kindergarten in August.
Kids, young kids like this, still need to nap.
And yes, as the others mentioned, 8:30 is late for bedtime for a 4 year old.
He is getting lack of sleep, from all angles.
Then, consider how long it takes, for his bedtime "routine." ie: bath, read books, brush teeth, sing lullabies and prayers.). That is a whole string of things of a routine, for bedtime.
With that routine, you'd NEED to start the bedtime routine, EARLIER. With my kids at that age, I began... the bedtime routine, 1 HOUR beforehand. AND then, also keep the room/house dark and only had 1 lamp on, made things quiet etc. So that, it set the stage, for bedtime. Making it, quieter and more subdued. I didn't do the bedtime routine, AT bedtime. I did it 1 hour, beforehand. So that, my kids went to bed, when it was bedtime. And so that, the bedtime routine, was not rushed.
Your son also plays out back, walks to school and back, and is in 1 sport activity. That makes any kid, tired. Overtired. And overtired kids... actually have a HARDER time, going to sleep. Or sleeping well.