How is finding the fourth vertex of a parallelogram that is not a rectangle or a square the same as it is for a rectangle or square? how is it different?

well, the only difference from a parallelogram that is a square or a rectangle and one that isn't, is that the interior angles are 90° in the square and rectangle, however for other ones, like the one in the picture below, they are not 90°.

however, getting to any vertex, is just a matter of moving horizontally and vertically the same distances, as you see those arrows on the picture, since both pair of sides are parallel, one moves the same distance horizontall and vertically to get to the next vertex, so on that, is the same as for a square or rectangle as well.

is only different in that you're not moving alongside the segment, in a rectangle or a square, the lines are only vertical or horizontal, so you simply move straight to get the next vertex, on other parallelograms like the one below, you do move the same but is not alongside the segments.