In this study, we develop a semi-automatic instrument-playing robot, Robo-Sax, which performs key fingerings automatically, while a human player performs the blowing motion. Additionally, we develop four types of Robo-Saxs (soprano, alto, tenor, and baritone) and propose a system that enables ensemble playing using a saxophone quartet. We seek to design the hardware and thoroughly develop the system software such that it can be utilized practically. Experiments using Robo-Sax are conducted with beginner, intermediate, and professional saxophonists to demonstrate this system's potential.