The Four Cards are a set of artifacts found in the ancient shrines of Ambrosia during Ultima III. These items proved essential to defeating the daemon-machine Exodus within its stronghold of Castle Death.
The prophet Radrion would tell the Stranger to seek knowledge of them in the twin cities of Montor, and there, the hero could learn that the cards were four in number and that they were to be inserted into some manner of panel. Later examination of the runes within the Dungeon of the Snake would reveal that these articles could be found within the four ancient shrines and that they should be used upon Exodus itself. The Stranger would eventually travel to the lost continent and collected these cards at the sacred sites which had been indicated. There, the card known as "Death" was found within the Shrine of Strength; the card known as "Love" within the Shrine of Dexterity; card known as "Moons" within the Shrine of Wisdom; and the card known as "Sol" within the Shrine of Intelligence.
It is not known precisely what the four cards were or what meaning, if any, their suits bore. However, the Lord of Time would eventually reveal to the Stranger that by inserting them only in a specific order --Love, Sol, Moons, Death -- could they hope to prevail against Exodus. When the hero and companions at last fought their way to the center of the Isle of Fire and found the bio-machinery which would accept the cards, they followed these instructions and tricked the hellspawn into enacting its own destruction.
It is not known what befell the four cards after Exodus' demise, although they survived the destruction of the daemon-machine and the subsequent sinking of the Isle of Fire. Centuries later, through means unknown, they would resurface, coming into the possession of Britain's Royal Museum, where they appeared in Ultima IX.
The Ultima 6 Project
Lycaeum in The Ultima 6 Project. As per Lorcan's instructions, the Avatar could use these cards to summon the soul of Exodus back to Britannia to facilitate the land's destruction.