In most households, I've noticed that "hell hour" is between 5 & 6pm, when the kids are tired and ready for a meal, but it is a little too early to feed them. Seeing as yours is still quite young, it makes sense that they'd need a break at around 4.
Maybe sit him down with a small meal and a quiet activity for 30 mins? I used to tape "Play School" as a settler for when the kids were overtired, and it gave them the break they needed to become human again.

Kerrie