The nurse is caring for a client with a sacral wound. The wound is full thickness, measures 4 cm × 6 cm with irregular borders, and is covered by a layer of black collagen. Which is this wound stage?

Number 4 is correct.

A full-thickness wound covered with black collagen, known as eschar, is unstageable. Wounds covered in eschar or slough cannot be staged because the full depth of the wound is not visible.