Ultima IV: Quest of the Avatar, first released in 1985 for the Apple II, is the fourth in the series of Ultima role-playing video games. It is the first in the "Age of Enlightenment" trilogy, shifting the series from the hack and slash, dungeon crawl gameplay of its "Age of Darkness" predecessors towards an ethically nuanced, story-driven approach. Ultima IV has a much larger game world than its predecessors, with an overworld map sixteen times the size of Ultima III and puzzle-filled dungeon rooms to explore. Ultima IV further advances the franchise with dialog improvements, new means of travel and exploration, and world interactivity.

In 1996 Computer Gaming World named Ultima IV as #2 on its Best Games of All Time list for IBM PC compatibles. Designer Richard Garriott considers this game to be among his favorites from the Ultima series.

Ultima IV was followed by the release of Ultima V: Warriors of Destiny in 1988.

Ultima IV is among the few role-playing games, and perhaps the first, in which the game's story does not center on asking a player character to overcome a tangible ultimate evil. The story instead focuses on the player character's moral self-improvement.

After the defeat of each of the members of the Triad of Evil in the previous three Ultima games, the world of Sosaria underwent some radical changes in geography: Three quarters of the world disappeared, continents rose and sank, and new cities were built to replace the ones that were lost. Eventually the world, now unified under Lord British's rule, was renamed Britannia. Lord British felt the people lacked purpose after their great struggles against the Triad were over, and he was concerned with their spiritual well-being in this unfamiliar new age of relative peace, so he proclaimed the Quest of the Avatar: He needed someone to step forth and become the shining example for others to follow.

Unlike most other RPGs the game is not set in an "age of darkness"; prosperous Britannia resembles Renaissance Italy, or King Arthur's Camelot. The object of the game is to focus on the main character's development in virtuous life—possible because the land is at peace—and become a spiritual leader and an example to the people of the world of Britannia. The game follows the protagonist's struggle to understand and exercise the Eight Virtues. After proving his or her understanding in each of the Virtues, locating several artifacts and finally descending into the dungeon called the Stygian Abyss to gain access to the Codex of Ultimate Wisdom, the protagonist becomes an Avatar.

Conversely, actions in the game could remove a character's gained Virtues, distancing them from the construction of Truth, Love, Courage and the greater axiom of Infinity—all required to complete the game. Though Avatarhood is not exclusive to one chosen person, the hero remains the only known Avatar throughout the later games, and as time passes he is increasingly regarded as a myth.

Instead of simply choosing stats to assign points to as in the first three Ultima games, players are asked various ethical dilemmas by a gypsy fortune-teller using remotely tarot-like cards of the eight virtues. These situations do not have one correct resolution; rather, players must rank the Eight Virtues and whichever stands as their highest priority determines the type of character they will play. For example, choosing Compassion creates a Bard, Honor a Paladin, Sacrifice a Tinker, and so on. This is also the first Ultima where playing as a human is necessary, eliminating other races such as the elves, dwarves, and "bobbits" found in previous games (although even in the first three Ultimas where they could be chosen as player characters, there were never any non-player characters (NPC) of those non-human races).