Projector, Signal, Ground, M4 is a pyrotechnic launcher about 12" long, like a mortar. See page 53 of the M8, M20 TM 9-743 dated 21 February 1944. On M-20 it's in a holder on the hull behind the driver seat. For the M8 is seems like there is a typo in the manual where it says where it mounts. Confusion with interphone locations? Maybe someone can tell us where it goes on the M8.
It shoots a variety of pyrotechnics labeled M17 to M22 which are cylindrical with a rod coming out on one end with stabilizing fins. A primer with firing charge is on the other end. The round is pushed into the tube head first, leaf springs holds it away from a fixed primer for safety, the soldier holds it up and slams it base-first onto a hard surface, round goes down and primer impacts firing pin, the thing explodes out the launcher fins-first and in about 100 feet it reverses and keeps going to 600 feet where it ejects the contents of the cylinder. These acted the same as the rifle-fired versions M17A1 and M17A1B2 up to M22 etc, etc, which were fired head-first from rifle grenade launchers.