"Four hundred dollars"
If 400 (чотириста) has a feminine ending, wouldn't the word for dollars also take the femenine form? (Долари)
400 doesn't have a feminine ending, and nouns never change gender (except nouns of people/animals with certain suffixes). 400 is a number. Only one and two have different endings for different genders, and it is the number that changes, not the noun following it.