Four of a kind, also known as quads, is a hand that contains four cards of one rank and one card of another rank (the kicker), such as 9&c 9&` 9. 9&e J&e ("four of a kind, nines"). It ranks below a straight flush and above a full house.
Your only way to beat four of a kind, or "quads" as they are often called, is to hold either a straight flush or a Royal Flush.