"throw them in jail and don't give them a sip of water, on the ninth day behead them!" eh, if these orders are really carried out, then you will only have corpses to behead since they die after 3-4 days from dehydration, maybe they meant just no food
and yeah, that fourth prince could be the oldest from the looks, at least he is definitely older than the third, so maybe the counting has something to do with their position in line dependent on their mother's status?