Mercenaries finally make their appearance in Diablo IV, with the Vessel of Hatred DLC. These battle brothers (and sister) will faithfully serve alongside your character and have a skill tree all their own. They may also be called upon in a purely reinforcement capacity, instead of accompanying your character at all times.
This Diablo 4 Vessel of Hatred guide outlines how to unlock access to all four currently available mercenaries.
Raheir
This blacksmith turned mercenary is essentially the tank, equipped as he is with a tower shield. Raheir can be relied upon to adequately defend and heal both himself and you, while your character engages in either close combat or ranged strikes.
Raheir can be unlocked by completing “The Hand Remembers the Blade” during the course of the main quest. Interact with him post-completion and he will become available to recruit at The Den.
Varyana
A close-combat mercenary equivalent to a Barbarian, Varyana is ideally teamed with ranged characters to complement their offensive capabilities, though melee characters can benefit from the pairing as well, via boosts to Attack Speed. Varyana can also ably defend herself, but less so than Raheir.
Varyana can be unlocked by completing the “Slayer’s Retribution” quest acquired via the Brutal Note pinned to a bloody stake in The Den.
Aldkin
Half-demon mage with a bevy of area-of-effect spells to call upon, Aldkin works best as support for close-combat characters who can defend him from attackers, while he handles crowd control with status effects like Vulnerable and Daze.
Aldkin can be unlocked by completing the “A Nameless Mystery” quest acquired via the Archive of Kuo Chosah found in The Den.
Subo
An archer with a variety of ranged area-of-effect attacks, mostly explosive, to rely upon. Subo is an effective support for melee characters holding the frontline in combat, while he manages crowd control with an array of status effects.
Subo can be unlocked by completing the “A Feather on the Scale” quest acquired via the Muddy Letter lying under a mercenary helm in The Den.
Those are the four mercenaries made available to hire in the Vessel of Hatred DLC for Diablo IV.