I had a Muscovy hatch a few chicken eggs once. It worked out okay, but the duck had had real ducklings before and seemed to know that these were different, and she wasn't very motherly with the chicks. They didn't follow her well either.
She looks like a normal (and very pretty!) 4-month old Muscovy to me! Muscovies develop their caruncles around 5 months and are fully mature and start laying around 6 months of age. I do have some adult Muscovies with very slight caruncling, so it's possible she won't grow much more.
My...