feorþa [OE]
ferthe [MI]
fourth [ME]
fourþ
https://www.etymonline.com/word/fourth#etymonline_v_40767
https://www.merriam-webster.com/dictionary/fourth
https://quod.lib.umich.edu/m/middle-english-dictionary/dictionary/MED15762
https://bosworthtoller.com/10456