Tier 4 refers to the set of class equipment that is acquired from the bosses of Karazhan, Gruul's Lair and Magtheridon's Lair. These sets, the first in The Burning Crusade, represent a vast departure from previous gearsets. Portions of this set are acquired from a half-size raid, instead of only full-size raids as both previous and later sets required. Additionally, for the first time some of the classes had multiple sets of gear to choose from when redeeming tokens; instead of each class having an assigned role and set gear being designed around it, hybrids and off-spec characters were given their own unique gearsets. Tier 4 equipment sets consists of 5 pieces, and the set bonuses are at the 2- and 4-piece levels.
Tier 4 can also refer to a guild working on the entry-level dungeons in TBC, although most typically this specifically references the 25-man raids instead of only Karazhan.
Druid: Malorne Regalia (Balance), Malorne Raiment (Resto), Malorne Harness (Feral)
Hunter: Demon Stalker Armor
Mage: Aldor Regalia
Paladin: Justicar Raiment (Holy), Justicar Armor (Protection), Justicar Battlegear (Retribution)
Priest: Incarnate Regalia (Holy), Incarnate Raiment (Shadow)
Shaman: Cyclone Regalia (Elemental), Cycone Raiment (Resto), Cyclone Harness (Enhancement)
Warlock: Voidheart Raiment
Warrior: Warbringer Armor (Protection), Warbringer Battlegear (Fury)
Tokens that drop from these bosses are one of the following: