Yea, good remarks. And especielly mini-kittens are pround for such suddenly collapses - and this 3-4 weeks seems to be the big dangerous passage. There are others, but this is the big.Having raised quite a few litter of orphans, I have had kittens die around that age, suddenly, for no apparent reason. I suspect that's the age that genetic diseases such as heart defects, liver shunts, etc. manifest themselves (although I've never had a necropsy done so I'm just guessing). So it is possible that he died of something like that (or even an accident)
I suppose the same danger is also for other kittens, normal sized, althoug it doesnt happen as often as for the minis.
Being runt, he was also more prone to have accidents happen to him, as everybody else was bigger and crawling around and on him.
So what happened we cant know 100% surely, but this here is the very most probable scenario. Like we speculated already earlier.