Peyton has to be sick. Either his brother gets to two wins, or Brady gets to 4. Either way, history will have Peyton as the odd man out. He can't be mentioned with Brady if Brady has 4 Super Bowl wins, and Peyton would have to win at least one more to catch Eli if Eli wins.
If Peyton doesn't retire, then I think he might have to go to another team and try to win another title or two or his legacy will slip quite a bit. History remembers the big winners in the big games.