You can make a case for all 4. I fail to see how Peyton and AP are more valuable to their teams than Brady or Rodgers. Consider that Peyton's D was much better than Brady's or Rodgers D. Peterson to me is a RB. Offensive Player of the Year for sure but his team had a worse record than the other 3 and it's because RB is not as important of a position.
How would Rodgers season be perceived if it went exactly the same, but the refs didn't blow the Seattle game and his D got a solitary stop in week 17 in the 4th quarter? He'd be 13-3 and coming off a 370 yard, 4 TD game in week 17 to nab the 1 seed in the NFC. 2 events entirely out of his control and totally irrespective of his own play. But because of that, Peyton is in the discussion and he is not. Peyton is now more "valuable" than he is. The award is kind of a joke when you break it down like that. Every year there are several deserving players and value means something different to everyone.
Congrats to all 4 guys for MVP caliber seasons. That said, Brady I hope you choke terribly in your first playoff game