d'Artagnan is a friend and associate of the Three Musketeers, and the protagonist of the novel. In the controversial sequel to the 1973 film (The Four Musketeers), it's presumably he that is referred to as the fourth musketeer.
(The Four Musketeers was controversial because the film was too long and so it was made into two; the sequel was released six months after the original. The actors were incensed that their work was used to make an entirely separate film, while they were only being paid for the work of one.)
© Haydn Thompson 2017