My boys are older but have ASD, so it's important to handle things right, for everyone's safety when one of them goes into meltdown. It's slightly different with each of the boys.
The thing that keeps my own adrenaline levels down is remembering that the meltdown or tantrum is often as a result of bewilderment or fear and that, no matter how angry they seem, they're actually feeling quite vulnerable at that moment. There is almost always something to be learnt from a tantrum or meltdown and, in a younger or immature child, it's almost never that they're out to deliberately piss you off.
DS1 can say some pretty awful things. They express the extremeness of his current feelings. There is no subtlety when he is distressed and he doesn't really get the nuances of feelings at the best of times. Many typical 4 year olds are probably emotionally on this level.
I try not to get sucked in. I don't engage unless it's really obvious that I need to engage, for safety's sake. Any discussion needs to happen afterwards. For now, I need to mentally and/or physically take a step back
If it's safe to do so, I keep my distance and leave the room if possible, after checking for anything fragile or dangerous that's likely to be thrown or knocked over. If it's just me and the two boys, I channel my energies into keeping the other one calm (and safe). If it's just me and the boy who is having a moment, I distract myself - Twitter and frivolous threads on mumsnet are good for this. If I'm feeling rattled, I might take it out on a random goady fucker.
I put my mental energy into working out what went wrong and how it's possible to prevent it from happening again, if it is possible. (Sometimes, the wind is simply blowing in the wrong direction and it was going to happen, anyhow). Often the signs were obvious, but missed.
If we're in public, I have my rictus grin and ultra bright and breezy face on. And apologise profusely and cheerfully to anyone who finds an angry boy lying on the floor in front of them.
Sometimes, there actually is humour to be found in the situation, because the whole thing is so damned surreal.
And wine.