If the assembly only calls for 3 fine but if there is a through hole for another, add it. No need to go to the dealer and pay an inflated price for a bolt and nut, your local Fastenal or Ace Hardware store will have the Metric / ISO bolt and nut / lockwasher in stock too.
Sure looks like it's moving from the paint mark / shadow. and I'd be checking the prevailing torque on all the mounting bolts, WITH A TORQUE WRENCH, not just by cranking down on them. Again, the torque values for different fasteners will be listed in your owners manual. The bolt head markings and shank diameter will determine what torque value they need to be at.