With equal length 4 bars, the angle of the pinion does not change, regardless of the suspension travel. It is a true parallelogram, If the leading pivot points of the 4 bars are vertically aligned, and the trailing pivot points are vertically aligned, at a "neutral position" (which in a perfect world, will be with full weight of body, engine, and chassis, resting on the springs, and the 4 bars running at 90 degrees to the vertical) the pinion angle will not vary either above or below the "nuetral centerline". There are other factors that are introduced when the top and bottom 4-bars are not of equal length, but as far as I know, we are not dealing with that here.