It's because it isn't a sentence. It's a phrase. The first word of a sentence is capitalized. A sentence, however, consists of a subject(noun) and a predicate(verb). "vier Mädchen" doesn't make a sentence- no verb. So if it said "vier Mädchen machen die Suppe.," then 'vier' should have been capitalized. Capitalization is not generally required for phrases, though. :) I hope that was helpful. If I am dreadfully wrong, someone please correct me.