a. - Next in order after the third; the ordinal of four.
a. - Forming one of four equal parts into which anything may be divided.
n. - One of four equal parts into which one whole may be divided; the quotient of a unit divided by four; one coming next in order after the third.
n. - The interval of two tones and a semitone, embracing four diatonic degrees of the scale; the subdominant of any key.