3 & 4 are one of the main characters of the Tim Burton animated film "9". 3 and 4 are twins, and the Historians of the group.
They are unable to speak, instead using flickering lights in their eyes to communicate with each other and 4 can project images from his/her eyes for the other members of the group. It is unknown what their genders are, but they are possibly female. Director Shane Acker has said that he 'leaves it to the fanbase to choose their gender'. They instinctively run and hide at the sight of danger.