Refer to the plumbing diagram and partial elementary school floor plan. WU, the developed length between the lavatory trap and common vent is 4`, and the lavatory is installed in compliance with the code, then the mi nimum allowable size of the trap arm is ____.

Refer to the plumbing diagram and partial elementary school floor plan. WU, the developed length between the lavatory trap and common vent is 4`, and the lavatory is installed in compliance with the code, then the mi nimum allowable size of the trap arm is ____.