The explosive child has some good strategies. Also the high needs child.
I would aim to have consistent routines so he knows what to expect, lots of warning of change of routine. Set bed and meal times, screens etc.
Don't expect more of him than he is capable of. Manage his environment if you know he is going to struggle with something preempt it and manage the situation before it escalates.
Look for early signs of an issue and distract/move forward to avoid it. Offer alternatives so for example toy throwing- give him some cushions to throw.
Make a list of the things he struggles with and pick a couple of them to work on at a time. Really break them down so say hitting -talk about being kind as a general rule and kind hands, redirect if he is going to hit, something like offering a cushion to hit or a ball to squeeze. Praise times he doesn't hit. Discipline if he does but then immediately move on and keep trying.
Massively lower your expectations to avoid being constantly on at him. Loads of reward/praise for behaviours you want to see.
Have one consistent discipline, personally I quite like a time out, if he's doing an undesirable behaviour, you (or other adult) take him out of the room . No shouting or telling off just sit with him for a few minutes in the hall or similar. Do some calm breathing , limit communication but give him a hug if he needs . When he's more calm go back in the room and try again. What you are teaching him is to regulate himself.
Role model the behaviour you want, when you get cross that you getting overwhelmed. And you're a grown adult responsible for your own behaviour. We as adults fund it hard to manage our emotions/frustrations at times yet we expect children to do it.
Your role is to advocate for your child and try to ensure they can manage their environment. Teach them strategies for when they can't and show them safe/acceptable ways to express their emotions.
It will be an ongoing process which sometimes will go well and sometimes not. Try to be on the same page as the other adults in his life. Don't compare him to other 4 year olds and think he needs to be doing xyz because he's four. It's your job to look at what he needs to learn and figure out how to get him there.
Don't fall in to the trap of thinking well he can do it at school or grandparents so he's just playing up. It's far more likely that he's masking his struggles at school and letting them out at home because he feels safe to do so or he's more distracted/stimulated at school or the the structure works better for him.