If Marquez is smart he walks away. The risk is all his in a 5th fight.
However, if he is fair, he takes it. Pacquiao gave him 3 rematches, arguably after winning the first 3 (but for a judging error in the 1st fight, that was a split decision for Pacman).
In the cold light of day, when he gets presented with a $20 million dollar purse plus PPV, I can't see him turning that down. He'll be on, at least, 50-50 in a rematch.