I'm sure someone will chime in but on my older vehicles (not sure if the C4 is old enough) I would have the door open at 45 degrees and lifting up on the back of it, see if there is any play in the hinges. If there is, you need new pins/bushings. If there is no play and it is solid, what we used to do is to put a block of wood under the door on the jam (so the jack would not touch the outside of the door) and use a car jack to lift it back up to get rid of the sag. I would feel comfortable doing this on my 81 Camaro but am not quite sure about this procedure on a C4 as I am not 100% sure of the construction of the door.