The four kings in a deck of cards are meant to reflect on the four horsemen of the apocalypse:
Suit
Horseman
Reason
Spades
Death
Because a spade is used to dig a grave
Club
War
Because you can hit someone on the head with a club
Diamonds
Famine
Because diamonds are beautiful but inedible
Hearts
Pestilence
Because marriage is like a hideous disease that consumes your body, corrupts your mind, and drives away your friends.
Behind the Legend
This legend for the names behind the kings in a deck of cards is just as spurious as the ones that say the kings represent four great historical kings (Charlemagne, David, Alexander, and Caesar), four of England's kings (Henry V, Henry VI, Henry VII, and Henry VIII), four great kings of legend (Midas, Tut, Kong, and Crab), or the four moods of Elvis.
There are also lists of names associated with the jacks (for example Be Nimble, Frost, Knife, and Nicholson), but again they do not have reliable historical accuracy. We will not discuss lists of queens, as this Web site may be viewed by young children or, more importantly, annoying activist groups.
So where did the four kings come from if they do not represent anyone in particular? In truth, they were originally intended to represent the "rulers of the four points of the compass." But that's such a boring explanation, that it's no wonder people come up with alternatives.