Charon is named after the boatman who ferried souls across the river Styx, Hydra is the nine-headed serpent who battled Hercules, and Nix is the Greek goddess of the night.
But P4 and P5 aren't really names, they're placeholders. That's where you come in. Over the next two weeks astronomers at the SETI institute are asking for your help naming the moons. It can't be just any name, though, they've suggested 12 names from Greek and Roman mythology and are asking for the public's input. If you don't like any of the 12 you can also submit your own idea. The voting is only open for two weeks. After that the votes will be counted and the suggested name submitted to the International Astronomy Union's nomenclature committee. So come on, head over to the website and help give the moons of Pluto a name!
Now let's just hope that 4Chan doesn't find out about this and try to make the name "Hitler's armpit" or something like that...