The OP specified that the four parent families in their scenario are all MMFF. So their reproductive capabilities would be exactly the same as two monogamous MF couples.Perhaps but there are many variables to take into account when determining what the individual composition of four parent families might be and how many children there might be within such a household. For example, families could be made up of FFFM, FFMM or FMMM if they wish to reproduce biologically. My scenario also did not take into account the availability of contraception of some sort which would also influence the make-up of a household nor those who are not biologically male or female.
How many children they would choose to have, assuming they can control their reproduction enough to make that choice, might correlate with how many parents there are. Perhaps two parent families would end up having more children because they keep having boys and they keep trying again for a girl, or vice versa, whereas in a four parent family, if one mother gives birth to nothing but boys and the other gives birth to nothing but girls, they still get to parent both. Or perhaps they'd have fewer because they want to keep the number of children down so inheritance doesn't get spread too thin. Or more because they're farmers and they need all those children to work the land.
In all of those cases, a couple would be likely to have more children than they would if they were part of a foursome.