If you have completed the game 4 times and upgraded all your weapons to level three, you can unlock Liberator level 4.

How: Go back to mission crash site. When you there, keep on the mission until you got a dead survivor and have been tasked to get his weapon.

Just simply pick up the weapon and it will be Liberator level 4.

This weapon has got a purple color, bullet explode like the human grenade (same grenade radius and same damage), increased clip size (120 in one clip), and the total ammo of the weapon is 1,200. It also has accuracy like a sniper rifle (without a scope).