So which is it... is he the 3rd or 4th Prince???
He's the third prince in English. But in chinese culture, you number the children entirely on birth order, gender is secondary. So, for example, here the eldest son would be 1st prince, then 2nd prince (rich kid), 3rd princess (pirate lady), 4th prince (our MC), 5th princess (little girl). They aren't numbered by gender, but entirely by birth order. Seniority, theoretically, matters more than gender, sort of.