part number 4 is one of four plastic self taping rivets that holds the mid top portion to the small aluminium bar (no. 3 in the diagram). They hold the shape of the bumper but not really give much support, if you are missing any one of the 4 rivets, you can see the holes on the bumper are much larger than the holes on the aluminium bar. If your bumper is loose on one side, I suspect you've identified the wrong missing part. It depends on exactly where it is loose and how loose, I suspect it is one of the 5 torx screws (no. 7 in the diagram) or more suspect, the mounting bracket where the screws goes on to.