You must be given with the point of the whole line. Knowing that you would have 4 equal parts, there must be 5 divisions. The first thing to do is find the midpoint of the endpoints of the line to create a mid-partition. You now have 2 equal parts. Next, find the midpoint again for the 2 equal parts. You make 2 new parts again for each side. The total would be 4.
To find the midpoint, just find the average of the corresponding x- and y-coordinates of the 2 points. If point A(1,2) and point B is (3,4), the midpoint would be (2, 3).