If we consider a flat, two dimensional graph, then the locus of all points that are at a distance of 4mm from the line AB comprises of two lines, one on each of the sides.
While if we consider a 3 dimensional space, the locus of all points that are at a distance of 4mm from the line AB will be a cylinder with a radius of 8mm.
It would be a shape with 2 lines either side and parallel to AB with perpendicular distances of 4 mm from AB, and lengths equal to AB; and it will have 2 semi-circles of radius 4 mm attached to the ends of the parallel lines.
