4 m
If we let h represent the height of the sail, then its width is h/2. The area of the sail is given by ...
... A = (1/2)wh
Filling in the given information, we have
... 4 m² = (1/2)(h/2)h
... 16 m² = h² . . . . . . multiply by 4
... 4 m = h . . . . . . . . take the square root