Handload the grenades into the four-round internal magazine.
Use the pump action handle to prime the mechanism.
Brace the weapon at the hip and squeeze the launcher's trigger forward of the main magazine.
The U4 Firebomb Launcher in Aliens: Colonial Marines is visually identical to the U1 Grenade Launcher unit fitted to Pulse Rifles as standard. Given that the Colonial Marines Technical Manual states the U1 is capable of using a variety of ammunition types, it seems possible the name "U4 Firebomb Launcher" is simply a gameplay designation, and that the weapon is in fact a U1 loaded with a different ammo type. The fact that the U4 is named after ammunition it fires rather than having a specific designation (unlike the U1) further supports this theory.