Chaos Undivided
Chaos Undivided is the in-universe theory of Warhammer 40,000 that worshiping (and seeking the favor of) all four Chaos Gods allows followers to draw on the strengths of all four Ruinous Powers without their weaknesses. It exists in Warhammer Fantasy as well, although it goes even further with the idea that the Four are merely parts of Chaos, with the entirety of it being an extension of all things both in the Realm of Chaos and in the material plane. In the latter interpretation even the Chaos Gods are pawns of fate in the grand scheme of things.
This theory was first postulated in 40k 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. Fantasy Chaos Undivided appears more as the secret truth to Chaos, and tends towards only representing the best among the best Chaos worshipers, with Everchosens uniting the varied mono-god groups unto a horde (that falls apart after their death usually).
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. The current Fantasy Everchosen Archaon, as well as Be'lakor, only see Chaos Undivided as the path to their goals; in the former case the destruction of all things, and in the latter to undivide Chaos and become it in it's entirety (a desire also shared by Nagash).
Followers of a single god (such as the World Eaters, Thousand Sons, Emperor's Children, or Death Guard) as well as most Warriors of Chaos 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 worshipers 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. Likewise the only Chaos Warriors that have ever achieved much of note in the Warhammer World are those who join the Everchosens in their journeys.
On the other hand, the greatest Champion of Chaos Undivided is Failbaddon and the most loyal servants are the Beastmen. Make of that what you will.
The most well-known to /tg/ follower of Chaos Undivided would be the fang-toothed 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 in 40k any more. This is fail as it would be a cool reason to be Undivided besides just trolling the single god lovers. It also would justify the existence of the Black Legion and World Bearers.
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.
Furthering the unfair gypping of Chaos Undivided is the newfound inability for Unaligned Chaos players to get anything nice. Both versions of Warhammer now demand a player Mark a Daemon Prince to one of the Four before it can be fielded, and even Black Crusade retconned that it's simply impossible for an Unaligned Heretic who reaches 100 Infamy to ascend to daemonhood unless he or she swears loyalty to Khorne, Slaanesh, Tzeentch or Nurgle in its Tome of Decay expansion. Any GM who understands that Lorgar and Perturabo are Daemon Primachs disregards this rule.