The Four Cards are found in the ancient shrines of Ambrosia in Ultima III. They are the only way for the party to destroy the machine part of Exodus in the Castle of Fire. This implies that they are either some kind of programming cards, or some kind of destructive hardware.
Stranded on the lost island of Ambrosia, the Stranger searched through the four ancient shrines to find the cards of Love, Sol, Moons, and Death. The stranger already knew that these cards could be Exodus' doom. But only after talking with the Time Lord in the Dungeon of Time, was the correct order in which to use the cards revealed. Fighting their way through the Castle of Fire, the stranger's party arrived at the machine core of Exodus. Inserting the cards in the right sequence, the machine exploded, spelling the end of Exodus and the loss of the cards.