Those 4 plates were intended to be support extension brackets for the body. They get mounted 2 per side, between the existing mounting point on the body and the existing bracket at the middle body bolt location (just behind the front doors)...they are pretty tight behind the pinch seam, you may have to bend it out a tad to get them on. Many people don't realize what they are really for, and subsequently don't install them... the truth is that they probably aren't all that important anyway.