Ultima 4: Quest of the Avatar

This was the first Ultima game to feature a real conversation system--whereas NPCs in the earlier parts would only give one canned answer when talked to, now players could interact with them by specifying a subject of conversation, the subject determined either by a standard set of questions (name, job, health) or by information gleaned from the previous answers, or from other characters. Many sub-quests were arranged around this.

The world of Britannia was first introduced here in full, and the world
map in the series did not greatly change any more from this point
onward.