fourthly

Concise Oxford English Dictionary © 2008 Oxford University Press:
fourth/fɔːθ/

▶ordinal number

• 1 constituting number four in a sequence; 4th.

• 2 (a fourth/one fourth) chiefly N. Amer. a quarter.

• 3 Music an interval spanning four consecutive notes in a diatonic scale, in particular (also perfect fourth) an interval of two tones and a semitone.

  ■ Music the note which is higher by this interval than the tonic of a diatonic scale or root of a chord.

– phrases
the fourth estate the press; journalism.

– derivatives
fourthly adverb.