Sanctuary Four was an inhabited planet. It was home to a sanctuary for the endangered trance worms. Access to this world was granted when a code, such as "Mandelbrot Set code nautilus", was accepted.
Cleveland Booker worked with the personnel at this sanctuary in saving the trance worms and, by the time he delivered Molly in 3188, he had successfully brought in at least one other trance worm.
In a conversation with Michael Burnham, Booker described his planet as being much like Sanctuary Four until he had to leave it. She had traveled with him to the planet when he delivered Molly. (DIS: "That Hope Is You, Part 1")