Oh, it's awful, isn't it? I have 4 DCs too, from 9 down to 3, and two hours after school broke up, they were bickering and whining about being bored!
They were much better after a good night's sleep, actually. They're still like toddlers in that respect: give them enough food and sleep and they do function better.
Making sure they have space from each other also helps a bit here. DC2 and DC1 are in that "alpha child" struggle constantly and DC1 nearly always wins, but DC2 is getting closer at times! So for one thing, perhaps they are learning something from it. But my real point here was that they both need to be out with friends of their own age for a while and get a break, or it's far too intense.
I go to help if I'm asked or if it sounds like someone's about to get hurt.
It seems to be an inescapable part of childhood and is so much worse with more DCs. I worked this out once:
2 DCs: 2 playing combinations: they either play separately or together
3 DCs: 5 playing cominations: separately, together, two and one
4 DCs: 16 playing combinations!!!
No wonder there's sooo much bickering going on in our households, OP - so many ways to be unhappy! (Why did we do it?! - but that's a thread for another day!)
So I guess expect it to be part of life and treasure those small moments when they briefly all get on!