I think the most likely reason women in the fourth group would have male cells would be through miscarriages. Most zygotes do not implant. Of those that do, about 30% miscarry before the woman even knows she is pregnant.
It is interesting that 80% of the women who had abortions had male cells in their bodies. Since only 50% of pregnancies are of males, that suggests they had more than one abortion. I wonder how many women would keep using abortion for birth control if they knew that their victims leave behind permanent evidence of their existence.
I found that interesting too but not entirely surprising.