I think age 4 is probably quite normal for them to begin to grasp what it actually means, even if the actual finality isn’t something they can fully understand yet.
My eldest was 3.5 when we had one of our horses pts. The year before when he was 2.5 one of our other horses died (very elderly). Both times I straight up told him X has died when he asked where they were. I let him see me be sad about it. I didn’t let him see their bodies or anything like that but he was aware they had died. He wasn’t sad and seemed to accept it. Both times, he told me repeatedly “X has died” and I always confirmed, yes they have.
My dad was horrified I’d told him and said I shouldn’t have. But my mum was always very matter of fact with me when I was growing up and I don’t remember feeling shocked or having a massive moment of realisation. So I feel very strongly I owe him my full honesty even if it is difficult.
When he was 4, my Nans dog died. I was straight up again and told him the dog had died. He sees my Nan weekly, he would notice immediately if the dog wasn’t there. Again, he accepted it and wasn’t sad.
But, this time he had more questions for me. And a few days later he had clearly been thinking about it and processing it because I had questions such as “do people die?” And “will you die Mummy”. Absolutely broke my heart in two but I still tried very hard to calmly answer his questions honestly. “Yes, everything that is alive dies eventually, nothing can live forever” and “yes, eventually mummy will die but it usually happens when people are very old and their bodies don’t work anymore”.
He asked me about it repeatedly for a few days and we did have some tears at some points. I reassured him it was ok to be sad and that I would answer any questions he had. That was about a year ago now and he has occasionally brought it up since out of the blue.
I have stood firm in my approach and I have not shielded my younger children from being present during these conversations either. They were there and would have heard my explanations to him but wouldn’t have comprehended what I said. I won’t shy away when the time comes to answer their questions too.
He hasn’t yet asked me what happens after things die or if he will die. I’m sure those questions are imminent too!
I think you are right to be honest OP, even if it’s hard. It will be harder if he gets to say age 6 and then realises.