Having a new baby in the house will certainly change things - this is true even without the whole quad/poly angle. Especially if they are first-time parents, for several months it is likely that they will feel they have time for very little else. Aside from offering to watch the baby from time to time so they can take a break, I don't have any solid advise to offer.
You may want to read AnnabelMore's blog on this forum. Although not a quad, she talks a lot about the effects of her (married) girlfriend's pregnancy and new-momhood on their relationship.
Me: poly bi female, in an "open-but-not-looking" Vee-plus with -
MrS: hetero polyflexible male, live-in husband (23+ yrs)
Dude: hetero poly male, live-in boyfriend (4+ yrs) and MrS's best friend
Lotus: poly bi married female, "it's complicated" relationships with Dude/JaneQ/MrS (1+ years)
+ "others" = FBs, FWBs, lover-friends, platonic G/BFs, boytoys, etc.
My poly blogs here:
The Journey of JaneQSmythe
The Notebook of JaneQSmythe