A flock of 4 year old hens are well and firmly entrenched in their respective places in the pecking order, the two Young Male Chicks are newcomers and they are viewed by your 4 year old hens as interlopers.
A 3 months old Male Chick is hardly a rooster, not even a cockerel really, but it is entirely possible for Young Male Chicks of that age to refuse to acknowledge defeat, resulting in your two Young Male Chicks meeting their deaths at the hands of your older hens. The fact that BOTH of your Young Male Chicks were found belly down is evidence that both of your Young Male Chicks were cowering from your hard bitten hens when they met their deaths and that they were NOT killed by a predator.