This is an excellent article written by Jason La Confora detailing why it would make sense for the Bengals to not pick at #4.
This strategy could make sense for the 49ers if Darius/Peterson/Miller are all taken by the time the #7 pick rolls around. They would need simply not take a player with their pick and let the next few picks happen. They could then pick up Amukamara a few spots later, at number 10 or so.
This strategy could save the club 8-10 million in guaranteed money that they could then use to resign key players like Franklin or Lawson.
How would the fan reception be to such a move?