Chaos Undivided
Chaos Undivided is the in-universe theory of Warhammer 40,000 (and, to a lesser extent, Warhammer Fantasy Battle) that worshipping (and seeking the favor of) all four Chaos Gods allows followers to draw on the strengths of all four Ruinous Powers without their weaknesses. This theory was first postulated by Lorgar, Primarch of the Word Bearers Legion; even today, the Word Bearers are still the primary followers of Chaos Undivided. although the Black Legion is also a notable follower.
Note that many followers of Chaos merely exploit the power of Chaos for their own good (or at least try to) and have no real religious affiliation with the gods. However, these groups are sometimes referred to as followers of "Chaos Undivided," although the more accurate term is "unaligned." This primarily applies to Traitor Legions like the Iron Warriors and Alpha Legion, as well as some renegades, like the Red Corsairs.
Followers of a single god (such as the World Eaters, Thousand Sons, Emperor's Children, or Death Guard) hold that followers of Chaos Undivided are weaklings that cannot choose a single god to venerate. It is true that followers of Chaos Undivided will never be held in the favor that single-god worshippers will; however, Legions of Chaos Undivided are also the only Legions that have not fractured into disunited warbands and are actually able to maintain the drilled discipline one would expect of the Space Marines.
On the other hand, the greatest Champion of Chaos Undivided is Failbaddon. Make of that what you will.
The most well-known to /tg/ follower of Chaos Undivided would be the fangtoothed woman who calls herself 'Cultist', with a close second being her long suffering guardian.
Chaos Undivided used to be thought of as a force of its own, not dependent on the Gods, but Aaron Dembski-Bowden has said that that isn't canon any more. This is fail as it would be a cool reason to be Undivided besides just trolling the single god lovers.
Chaos Undivided had its own Mark for about one edition in both 40K and WFB, granting the ability to reroll Psychology (WFB) or Morale (40K) checks; a handy ability, but which got dropped in all subsequent editions.