A derailleur utilizes a 4-bar linkage, including a housing which acts as one bar of the 4-bar linkage. A hydraulic assembly located within the housing moves the housing relative to the mounting shaft in response to hydraulic pressure to define a chain guide operating path. The derailleur device may include a detent mechanism formed on one of the three other bars. In one embodiment, the 4-bar linkage includes an input shaft rotatably attached to the housing at a one end, and an output shaft rotatably attached to the housing at the other end, and the housing rotates around the first shaft in response to the actuation force. The hydraulic assembly may include a piston which moves within a cylindrical bore, with one of the three other bars of the 4-bar linkage being attached to the piston. The resulting hydraulic derailleur design is comparable to mechanical (cable-pull type) derailleurs in size and weight.