You have not given the year of your car but in the Haynes manual for the 2004 to 2010 model C4 under the section for door refitting (page 11.8) it has
6 Refitting is a reversal of removal. Once the door is fitted, adjust the striker plate so that the door opens and closes easily but firmly. With the door handle pulled out, shut the door and check that the lock slides over the striker plate without scraping.
Before you slacken the striker plate torx screws, I would mark its position so it is easy to check how much it has been adjusted or to put it back in the original position. As well as adjusting the striker plate, you could check for wear in the door hinges which may have allowed the door to drop slightly.