What part of speech is the italicized word in the following:

four balloons floated away into the sky.

note: the italicized word is (four).

you may also refer to the text material exam question #7 for the italicized word.

Which topic or issue would be most appropriate for a four- to five-page research paper? teen health in the us how a friend recovered from anorexia national eating disorders association teen athletes who suffer from eating disorders