The singleplayer campaign of SAS: Zombie Assault 4 is based on a storyline, taking place in less than 1100 years from now, or to be more precise, in the year 3104. The setting is mostly in Aartis, but some maps are set in other locations.
About 1000 years into the future, the zombie virus from Planet Thera spreads to the Kurios Spaceport Control Station in the Trans-system. The crew of Mantis 12 cannot take off because the ramps at Bay 10 are malfunctioning. Combot requests the pilot to take off now because a high-level biohazard had been detected in Bay 5 of the North Wing. Then, the virus spreads throughout the station. The zombies come in to the dropship at Bay 10, and the SAS are ordered to defend the ship until it takes off. When the ramps are finally repaired and the ship ready for takeoff, a giant mutated zombie (the boss) enters the ship, and the SAS must kill it before takeoff can commence.
A Jolt Ship has been sent into space to completely destroy the Planet Thera because it is lifeless and full of zombies. The Trans-system President confirms the strike, but Combot, an electronic system that controls all computers and robots on Aartis, states that the infection has spread to Planet Aartis, where the human race resides. It has located a vaccine, but the orbital strike was already confirmed and cannot be shut down because the countdown had reached 30 minutes. Then, the SAS have been sent to the surface of Thera to retrieve the vaccine before the bombardment commences. Afterwards, the SAS takes the Vaccine to a dropship, which takes off. After a few moments, the Jolt Ship destroys Thera and everything on it.
The virus had spread to Aartis and infected over 600,000 people. Some survivors have barricaded themselves in their homes and shops, but the SAS must try to rescue them all. The SAS must also kill each and every zombie in the area. After they were evacuated, a large mutated zombie attacks them, which is the boss. Then, the SAS finally defeats all the zombies in the city.
A new surprise had shocked the Trans-system. A biological infection has also become a digital pathogen, when suddenly, the zombie virus infected electronic equipment, including Combot. Machines and humans have been harvested into pods and the SAS must defeat them at the supermarket. The SAS must also eradicate the mutated Zombie Pods, which have grown into Minion and Boss Eggs. After doing so, the SAS evacuates in a victory.
A VIP house has been invaded by zombies and is hiding in his home. Like Survivors, the SAS must save the VIP, guiding him to a helicopter site. Zombies lurk in the hallways and in the front gardens, where there are also Zombdroid Servants. These Zombdroids appear to be the VIP's servants infected by the digital pathogen, and what is believed to have allowed the virus to travel from human to machine, due to their cyborg composition. There are also Zombdroid Soldiers. They appear to be the VIP's armed personal guards infected with the digital pathogen.
A hospital with survivors has lost power, disabling its defense system, and the SAS must restore it through repairing switches in underground tunnels. As they turn on the switches, infected Combot activates turrets and Loaderbots to make an attempt kill the SAS, if they win one of the nurses will thank them.
The last survivors from Aartis have been gathered at the market square, ready for evacuation. Zombies are attacking the place, but the civilians don't have enough time to effectively barricade the zombies out, so the SAS are sent to defend the civilians from the hundreds of zombies.
The SAS has lost contact with a mining colony on Boreas. They suspect the planet may be infected, but they don't know how. By the time the player arrives, HVM has already sent several mercenaries who were sent to assist local government there. The SAS is tasked with finding out what they can. The SAS soldiers arrive at the station and meet the HVM Captain and his soldiers. After fighting waves of zombies the HVM Captain and the soldiers fire at an ambulance, destroying it. When one of the SAS soldiers ask why they did so, the HVM Captain responds saying that they have their orders and that there can be no witnesses. The SAS soldiers are forced to kill the HVM Captain and his soldiers. The SAS then evacuates on a truck. During the ride, lots of zombies swarm on the way, attempting to destroy the truck. The SAS are now tasked with killing the zombies in order to defend the truck. Eventually, the truck is destroyed in an avalanche. When the SAS gets off the truck, they meet a boss zombie along with various groups of weaker zombies. The SAS must kill the boss zombie to complete the mission.
The SAS soldiers arrive at a nuclear plant due to the fact that it has lost control and must be shut down manually. The control has gone haywire, causing the doors to open and close at random times and rooms that can fill up with radioactive gas. The SAS must manage to avoid those rooms and power down each reactor manually and are given 5 minutes to do so for reach one. After shutting down all reactors, they restart the main computer and are attacked by multiple boss zombies along with weaker groups of zombies (that spawn continuously until the boss is killed and the countdown timer hits zero).
The player must defend the ship from zombies coming in. He/She must defend the ship for about 6:30 minutes. Zombies enter from one side, attacking the ramp doors. At about the 00:30 mark, the boss(es) comes out, and other zombies do not stop spawning until the time hits 0:00.
The player must clear the terminal from zombies, then get on the subway to get to the lab. After extracting the vaccine, he must head for the dropship. The time limit decreases from 14 minutes, depending on rank. Then the timer reaches five minutes, a message is sent to the player to hurry up. Also, when the timer hits 2 minutes, the game reveals which room has the vaccine. A boss guards the dropship, along with lots of zombies.
The player must kill all zombies in the city. Survivors are located in various buildings, but not all of them, either they could have loot or zombies. The player can stand in front of any door to break it down, revealing the inside of the building. After all sixty civilians are identified as "Rescued" or "Killed", Combot detects large tremors underground. As the player go back to the starting point, a boss emerges from underground. The SAS are told to distract the boss while the others evacuate the survivors.
The SAS must destroy the pods and purge nests in the market. Then they reach a parking garage where there are Minion Eggs and a Boss Egg, which contains the boss.
The player must find the VIP in one of the hiding spots, which are five rooms around the house. Then, the player must escort him to the helicopter site. Zombdroids spawn at the two front gardens. Only the Zombdroids spawning near the helicopter site need to be killed in order to evacuate.
The SAS must activate the switches, which, in order, are Alpha, Bravo, Charlie and Delta. While the SAS fixes the switches, Infected Combot activates turrets and Loaderbots. Various types of zombies spawn during this mission, including Zombdroids. As the Loaderbots are activated, Runners and Spitters spawn non-stop in the center room. Also, each time the mission is played, the location of each switch is random.
Because most of Aartis was infected, the last survivors of Aartis must be evacuated or else there would not be any more Aartians. There are eight waves of zombies to survive, each with different zombies for each game played. The last wave always has a boss or a few bosses, but players must be aware of the other zombies, and not let them get past. If too many zombies reach the end, the city will be overrun, initiating a failure.