Kerberos (formerly known as S/2011 (134340) 1 and informally as P4) is a small moon of the dwarf planet Pluto. The satellite was discovered on 28 June 2011. It is the fourth moon of Pluto to be discovered. The others are Charon, discovered in 1978, Nix and Hydra, which were discovered in 2005, and Styx, discovered in 2012.