A protected planet to which Cleveland Booker delivered a rescued trance worm in 3188, and where he and Michael Burnham later made their home beside a transplanted Kwejian world root.
Sanctuary Four is a refuge planet of the 32nd century, one of several worlds where endangered species are guarded from poachers. In the Discovery episode That Hope Is You, Part 1, Cleveland Booker reveals that the cargo he stole back from another courier is Molly, a trance worm, and he and Michael Burnham release her there into a lake.
The visit shows Burnham that Book is more than a smuggler and introduces her to the network of people still working for something beyond profit after the Burn.
The planet returns in the series finale Life, Itself. A cutting of the Kwejian world root, preserved in the Eternal Gallery and Archive, is planted on Sanctuary Four, and the closing scenes show Burnham and Book living there decades later.