To quote "Play according to Hoyle: Hoyle's Rules of Games" (Penguin Books USA, 1983), from the poker section:

"Four of a kind--any four cards of the same rank. As between two such hands, the four higher cards win. Four of a kind beats a full house or any lower hand."

In a standard game, four of a kind is beaten only by a straight flush. In a game with wilds, five of a kind also beats it (and a straight flush).