Meaning of 'We're a Four'

This had nothing to do with the size of the group but their sport--rowing. A "four" is a 4-man boat. It can come with or without a coxswain (in this case with, as the girl was their coxswain). A coxswain or "cox" for short is the person who sets the speed of the boat by giving a beat for the rowers to row to, speeding up or slowing down as the needs of the race dictates. They are the main tactician in the race and the rowers depend on the cox to win. This person is usually small and lightweight, so that the boat is as light as possible.