Level 4 is a Campaign level in which the Marine fights his way through a series of outdoor Civil Security fortifications. In level 4, three things are introduced, the Hound Walker-CS, acid, and Lite Railguns.
In Level 4, the Marine exits the Usurpation base and finds a Drone. After killing the Drone, the player faces off against three Minor Usurpation Soldiers and an Advanced Usurpation Soldier. However, the aliens are also fighting against a Hound Walker-CS. After the Marine destroys the walker, a Civil Security Lite ejects out, armed with a Pistol CS-Pro.
After swimming across a small stream, the player fights another Hound Walker while being fired at by units with Rocket Launchers, Lite Railguns, and Assault Rifles. The player must proceed to the Civil Security bunker and clear it of the Lites and Heavies, picking up a Railgun in the process. Behind the bunker is a Civil Security base. There are several Heavies and Lites guarding the base.
The player is first introduced to acid, as it is a pitfall at the end of the level under the elevator and jump pad. After going up an elevator or using the jump pad, you reach the Level Clear area and the level is completed.
- Minor/Major Usurpation Soldier - Found at the beginning, escaping a Hound Walker, there are 3 Usurpation Soldiers.
- Advanced Usurpation Soldier - Found with the Usurpation Soldiers, there is 1 in this level.
- Civil Security Lite - There are 13 Lites, appear with Assault Rifles and Pistols.
- Civil Security Heavy - There are 8 Heavies, using heavier weapons such as the Rocket Launcher CS-LitBro and the Lite Railgun.
- Hound Walker-CS - There are 2 Hound Walkers, each piloted by a Civil Security Lite with a pistol. Both have 150 HP, so they are either battle-damaged from fighting off the Usurpation Forces, or weaker models sent to defend the small base.
- Drone - There is a Drone at the beginning of the level. It fights on the Usurpation Forces side.
- You can hide and let the Drone shoot grenades, while the grenades fly over you and hit Civil Security soldiers and Usurpations instead. However, this only works on Normal and Easy difficulties; on Hard and Impossible, in which enemies may not shoot at their allies, Drone's grenades would fly through Usurpations, hitting Civil Security forces only.
- It's the only time a Drone is seen attacking Civil Security. Normally, Drones are seen attacking Usurpation Forces instead (like in levels 1 and 13), and considered as a part of Civil Security forces. It is possible that the Drone is malfunctioning, shooting Civil Security instead, or possibly hacked by the Usurpations.
- There is a gravitator lift that you can use if you fall off of the final elevator.
- This map has the map size of 18491 bytes.