Might be a bit controversial but I think a fifth installment has potential, assuming the knockout hasn't ruined Pacquiao.
1. It was an emphatic victory/KO but prior I regarded it as possibly Marquez's weakest performance against Pac up to that point.
2. Pacquiao has given Marquez rematches to try and even the score so if Pacquiao wants it, maybe Marquez should do the same
3. Another Pacquiao loss, especially if it was another KO would unambiguously mean retirement, "three strikes and your out" and all that shit.
4. I'm not saying Marquez was on PEDs or not, but some are, and if a rematch involved proper drug testing protocols (VADA etc), the doubters would regard it as a "clean" fight, and people generally would be intrigued to see what difference there is. Marquez himself might be hurting from unfair allegations and want to prove he wasn't cheating.
5. It's Pacquiao and Marquez FFS, the gift that keeps on giving. Two great fighters fight, they both make a ton of money and fans could end up witnessing another classic.