The 4th Founding of Space Marines occurred several solar decades after the disastrous events of the War of the Beast ended in 546.M32 -- the massive WAAAGH! of the Ork Warlord known only as The Beast -- which laid waste to much of the known galaxy and threatened to destroy Terra itself.
Following the successful eradication of the massive greenskin invasion force and the defeat of its Warlord, the Lord Commander of the Imperium Maximus Thane, the former Chapter Master of the Fists Exemplar, and now the newly-appointed commander of the reconstituted Imperial Fists, decreed that the Imperium would raise as many new Chapters as they had gene-seed in the great vaults of Terra to sustain.
All Chapters of the Adeptus Astartes were ordered to release a portion of their Veterans to provide the initial basis for this Founding, the greatest since the First Founding. Gene-seed tithing would be doubled for the next standard century to replenish the vaults. When it was done, there would be hundreds of Chapters to ring the Imperium in adamantium and ceramite instead of dozens.
After enough gene-seed had been collected in the Mechanicus vaults on Terra, the High Lords of Terra authorised the 4th Founding of the Adeptus Astartes, sometime in the middle centuries of the 32nd Millennium, to help bring order to an Imperium rife with civil strife and insurrection, and to help restore Imperial law.
Soon after the War of the Beast, the Grand Master of Assassins Drakan Vangorich deployed the elite Assassins of the Officio Assassinorum to slay all of the other High Lords of Terra. Afterwards, the Imperium descended into a period of anarchy, and was ruled by the iron hand of Vangorich for nearly a standard century.
In ca. 626.M32, nearly eight solar decades after Vangorich seized total control over the Senatorum Imperialis, two of these newly created 4th Founding Chapters, the Halo Brethren and the Sable Swords, fought alongside the newly returned crusading Imperial Fists Chapter, to help forcibly remove Vangorich, who had become a mad tyrant, from power.
Notable 4th Founding Chapters
|Halo Brethren||Unknown||Unknown||Unknown||The Halo Brethren are a 4th Founding Chapter created from an unknown lineage sometime during the middle centuries of the 32nd Millennium. They would fight alongside their newly created cousins, the Sable Swords Chapter, and the Imperial Fists, to help forcibly remove the mad tyrant Drakan Vogorich from power. There is little other information on this obscure Chapter in official Imperial records.|
|Imperius Reavers||Ultramarines||Roboute Guilliman||Praedis Zeta||The Imperius Reavers are a 4th Founding Successor Chapter of the legendary Ultramarines, created during the 4th Founding of the 32nd Millennia. The Chapter's homeworld is the Hive World of Praedis Zeta in the Vidar Sector, which is located in the Eastern Fringe of the galaxy at the farthest edge of the Ultramarines' Realm of Ultramar.|
|Sable Swords||Unknown||Unknown||Obsidia||The Sable Swords are a Loyalist Space Chapter created in the latter years of the 41st Millennium, making them one of the newer Chapters Founded within the Adeptus Astartes. They would fight alongside their newly created cousins, the Halo Brethren Chapter, and the Imperial Fists, to help forcibly remove the mad tyrant Drakan Vogorich from power.|
|Tome Keepers||Ultramarines||Roboute Guilliman||Istrouma||The Tome Keepers are a Loyalist Space Marine Chapter and successor of the Ultramarines, raised during the 4th Founding shortly after 546.M32. They call the Desert World of Istrouma home.|
- Adeptus Astartes: Successor Chapters (Limited Release Booklet)
- Deathwatch: Honour the Chapter (RPG), pp. 89-96
- Imperial Armour Volume Two (Second Edition) - War Machines of the Adeptus Astartes, pg. 17
- Imperial Armour Volume Ten - The Badab War, Part Two, pp. 13–14, 19, 60-61, 69, 86, 114-123, 127, 134, 178
- Imperial Armour Volume Thirteen - War Machines of the Lost & the Damned, pg. 21
- Index Astartes "Emperor's Shield - Space Marine Chapters of the Armageddon War," by Chris Cook and Pete Haines
- The Horus Heresy - Book Three: Extermination, by Alan Bligh, pp. 134, 138, 140
- Warhammer 40,000: Campaign Weekend - Vidar's Fate: Praedis Zeta (Campaign Supplement)
- White Dwarf 248 (UK), "The Third War for Armageddon", pg. 76
- White Dwarf 101 (UK), "Index Astartes", pg. 71
- Angels of Death - Bitter Salvage (Short Story) by Nick Kyme
- Angels of Death: The Judges, In Their Hunger (Short Story) by David Annandale
- Devil's Trappings (Short Story) by Nick Kyme
- Salamander (Novel) by Nick Kyme
- Legacy of Russ: The Lost King (Short Story) by Robbie MacNiven
- Vengeful Honour (Short Story) by Nick Kyme
- In the Depths of Hades (Short Story) by Nick Kyme
- Machine Spirit (Short Story) by Nick Kyme
- The Beheading (Novel) (Book 12 of The Beast Arises series) by Guy Haley
- White Dwarf 458 (November 2020), "Flashpoint: Argovon Campaign", pp. 23-25, "Assault on the Nurtheos Shore," pg. 26, "Index Astartes: Tome Keepers" by Dan Harden & James Gallagher, pp. 46-59
- Third War for Armageddon Worldwide Campaign - Forces Disposition, Imperial Forces: Marines Malevolent