Designer Richard "Lord British" Garriott had already made a name for himself with the first three Ultima titles. These turn-based role-playing games borrowed rules and mythology from Dungeons & Dragons, infusing them with innovative plots involving space and time travel. They were definitely significant works, and by converting the traditional tabletop experience of D&D to home computers, they paved the way for the PC and console RPGs we know today. But aside from the technological feat of computerizing complex RPG rules, they were hardly revolutionary.
Ultima IV, on the other hand, most definitely was. Not through its technological advances, since it looked hardly better than Ultima III. And not through its fundamental gameplay, since it played like only a slightly evolved version of what Ultima had been doing all along.
No, Ultima IV was a revolution simply because of its theme. Your goal here was not to save the world from an evil magician or a diabolical computer; your goal was to save the world from itself. Lord British, the benevolent ruler of Britannia, notices that his people are lacking in direction and focus in their lives, and puts out a call for a leader to step forth and serve as an example of virtue to the populace. If you wish to become this leader--this avatar of virtue--your job is to learn, study, and implement the Eight Virtues: honesty, compassion, valor, justice, sacrifice, honor, spirituality, and humility.