What was the name of the 4th musketeer?

D'Artagnan is the character who is appointed to the Musketeers at the end of the novel "The Three Musketeers." Though he spends much of the book as a member of a less prestigious military company, d'Artagnan eventually becomes the fourth Musketeer.

At the beginning of the novel, d'Artagnan loses a letter of recommendation that would have allowed him to join the Musketeers outright. Without it, he is not able to enroll in the company. However, over the course of the novel, he helps uncover and thwart a plot to assassinate the Duke of Buckingham. His actions earn him a promotion to the rank of lieutenant in the Musketeers.