Some 4yos do have meltdowns which are very similar in severity to those of children with SN, without actually having SN (or the potential to grow into violent adults).
At least 3 members of my family did (though in two of those cases it was possibly related to childhood trauma).
All three children were normally well behaved children, noted for their caring attitude most of the time, who would then have moments when they just turned into something different.
Two are now grown up, one is a teen, none of them have ever been in trouble at school or outside the home. But their tantrums/meltdowns were truly spectacular.
My mother, SIL and I, who had to deal with the respective children, found that disciplining after the event was pointless: during meltdown the child would get carried away to the extent where they simply had no recollection of what they had done afterwards; it would be like disciplining me for something stupid I may have said when under the influence of a GA.
What was essential during the outburst was restraint and the firm reassurance that "I will not let you hurt me, I will not let you hurt anyone". They needed to know that precisely because they were frightened by their own lack of control.
I don't think I'm a lax or timid parent in any way and I have never had a problem with disciplining my children in the ordinary course of things. But disciplining dd out of her tantrums - no, I don't think that could have been done, at least not without escalating the discipline to the point of outright abuse. Firmness and calm did better.