LMAO that would be fucking wild. Honestly I think a lot of it depends on the individual and how they conceive. Kauris and Urale are both quad folks that have maintained elfshape for literally centuries, and Kauris gave birth to just a single kid, who yes, probably came out as a little pup. (Nadir was basically "cursed" by Urale as an infant to be trapped forever in elfshape, which has lead to a whole host of health problems for her.)

I do think that it's the norm for quadfolk pups to be born in their natural quad state though, regardless of what form the parents primarily take, and I think it's common for quadfolks to have litters. Again, depends on the species, but I think most litters are pretty small. It's more likely to get twins and triplets than larger litters.