The MK 4 fires one grenade each time the fire key is pressed. Grenades will either explode upon contact with an enemy or three seconds after it has been fired. Grenades will bounce against a wall if it hits one. If a grenade fired by the player touches you, it will explode upon contact, which you can take damage from.
The MK 4 can be charged up, which will affect how much damage the grenade will do and how fast it will be when it is fired. If fired without charging up, the grenade will be about as powerful as a rocket and will not go very far, but a fully charged grenade will do more damage than a rocket and will go very fast.
- The MK IV's grenades significantly bounce more than the MK III's, making lobbing grenades around corners much more useful and practical because of the better physics system.
- Charged up grenades can be very useful against medium-strength foes because of its power and speed, but the player should remember that, if a grenade misses, there is a chance of the grenade flying back because of how fast it is.
- Like the Serious Sam 1 version of the MK 4, uncharged grenades can be used to take down tightly-packed groups of weak enemies, such as Orc Grunts.
- Charged up grenades are not recommended to be used in tight spaces because of the possibility of splash damage and because how fast the grenade is. A fast grenade bounces around longer, making a missed shot very dangerous.
- The MK 4's ammo model is the same as the model used for the MK III's ammo in Serious Sam 1.
- MK III Grenade Launcher – The Serious Sam 1 equivalent of this weapon.