x1 = 200
y1 = 400
x2 = 700
y2 = 200
dist = 20

x3 = (x1 + x2) / 2
y3 = (y1 + y2) / 2

a = x2-x1
b = y1 - y2

norm = sqrt(a*a + b*b)
a = a / norm
b = b / norm

x4 = x3 + a * dist
y4 = y3 + b * dist