Not many knew, in fact a total of three humans, three aliens and a werewolf, that there were actually five marauders. The names of the four you know are;
Remus Lupin and
But, in addition to these four, there was a fifth. His name was David Tennant. Although, amongst the five of them, they all went by different names;
Remus was Moony and
Peter was Wormtail.
For those of you who don't know, they are called this because Remus is a werewolf and Peter's animagus is a rat. The Marauders became animagus after they found out about Remus being a werewolf so they could keep him company, and out of trouble, during the full moon, as werewolves only attack humans.
However the other three members all went by two different names. Amongst the five they were;
James was Prongs,
Sirius; Padfoot and
David was Whiskers.
Again the names are for animagus forms. James was a majestic stag with huge antlers, hence Prongs, Sirius was a large black dog and David was a cat. But the three of them had a different set of names, in fact some people who over heard these names seemed to think they were more of an identity.
James was Wizard,
Sirius was Prankster and
David was... Doctor.