User blog comment:S.C. Amigo/Theory on the 4th Yonkou./@comment-95.34.110.111-20100618122044

Many have said that as all the other yonkous are men, the last one will be a woman. Well what if, even crazier than Lola's mom, the fourth yonkou is Luffy's mother.

I mean, we haven't gotten any information as to who Luffy's mother is, only his fathers side of the family. So it could be possible, as well as quite fun.