Serious Sam 4

As in other entries in the series, Serious Sam 4 involves fighting against massive hordes of enemies in large areas. Enemies either attack close range or from a distance.

The player does not have a weapon limit, and is able to carry an infinite amount, such as shotguns, a minigun, grenade launcher and sniper. Reloading returns from Serious Sam 3: BFE, but the weapons have been adjusted, making this less of a necessity. The knife from the Encounters returns, replacing the sledgehammer.

An unknown amount of time before the events of Serious Sam 3: BFE, a massive portal opens in Tunguska, Russia which serves as a bridgehead for Mental's forces to invade Earth. As the whole planet scrambles to repel the alien invasion, a unit from the Earth Defense Force led by "Serious" Sam Stone is sent by General Brand to Rome to meet Father Mikhail, a priest who claims to have found some clues leading to the Holy Grail, which, according to him, is a powerful alien artefact hidden inside the Ark of the Covenant that may aid them against the invasion. However, the team is forced to abandon the city when it is overrun by the army of Lord Achriman, one of Mental's generals. In order to get back to the city, Sam and his companions retreat to Pompeii, where they remotely use the seismic capabilities of the HAARP complex to trigger an eruption on Mount Vesuvius that disrupts Achriman's air forces. Jones, one of Sam's companions, is killed by one of the alien monsters during the mission.

Upon returning to Rome, Sam meets a gun-toting old lady called Nonna who provides him directions to the Vatican, which Father Mikhail believes contains the next clue to the Grail. There, Sam retrieves a book that reveals the Grail is located in an abandoned church in Carcassonne, France. In Carcassonne, Sam is confronted by Achriman, who kills Kenny, another one of Sam's friends, in front of him before fleeing. With help from the local resistance, Sam and his friends arrive at the church and defeat Lord Achriman, whom Sam kills personally to avenge Kenny.

Development of Serious Sam 4 began in October 2012. The game was confirmed in June 2013, when the website Humble Bundle launched a "Humble Weekly Sale" containing several past games from the Serious Sam series to help fund the game. While playtesting the game's "jammer" feature—a movable device that could disable objects in a certain radius—the development team devised that this component and further prospective mechanics would better fit a different type of game, which was developed as The Talos Principle and released in 2014. In September 2015, The Talos Principle co-writer Jonas Kyratzes and his wife, Verena, were announced as the screenwriters for Serious Sam 4.

Serious Sam 4 was announced under the name Serious Sam 4: Planet Badass in April 2018, during the opening of the Reboot Develop 2018 conference in Croatia, with a teaser trailer. One level was demonstrated behind closed doors at E3 2018 that June. At the time, the game was planned to launch in 2019. In May 2020, Serious Sam 4 was re-announced without the Planet Badass subtitle and with a release window of August 2020 for Microsoft Windows and Stadia. According to developers, the Planet Badass title, which had borne out from the game's plot of a few Earthlings fighting against invading forces, was dropped as localizing this part of the title to other languages was impossible to do.

Serious Sam 4 received "mixed or average reviews", according to review aggregator website Metacritic. The site calculated normalised ratings of 68/100 for the Microsoft Windows version, 65/100 for the PlayStation 5 port, and 57/100 on Xbox Series X/S. Fellow review aggregator OpenCritic assessed that the game received fair approval, being recommended by 38% of critics.