If the slope of line C is -4, the slope of line D, which is perpendicular to C, is the negative reciprocal of the slope of C: -( 1 / (-4) ), or +1/4.
Note how 1/4 is the native reciprocal of -4, and that -4 is the negative reciprocal of 1/4.
The slope of line D is 1/4.