KöMaL 2015 B4691

$a$ , $b$ , $c$ and $d$ are four parallel lines in the plane, in this order. The distance of $a$ and $b$ is 1, the distance of $b$ and $c$ is 3, and the distance of $c$ and $d$ is also 1. Consider the rectangles that have exactly one vertex on each line. How can we obtain the rectangle of minimal area, and what is this area?