Who was the fourth Musketeer?

That would be d'Artagnan.

From the story "The Three Musketeers" by Alexander Dumas.

The fourth Musketeer was d'Artagnan, the main character of the book "The Three Musketeers" by Alexandre Dumas. He isn't a Musketeer yet at the beginning of the book, he's just an applicant. When he first meets Athos, Porthos, and Aramis (the "Three"), they end up dueling. The Cardinal's guards arrive to break it up. D'Artagnan and the 3 Musketeers join forces to fight the guards. D'Artagnan becomes friends with Athos, Porthos, and Aramis and later becomes a Musketeer himself.