Where does “IIIIth” come from?

IIIIth (Middle English) comes from Middle English ferthe, from Old English fēorþa, from Proto-West Germanic feurþō, from Proto-Germanic fedurþô — fourth.

Ancestry of “IIIIth”, step by step

StepLanguageWordMeaning
1Middle Englishferthefourth; One of four equal parts of a whole; a...
2Old Englishfēorþafourth
3Proto-West Germanicfeurþōfourth
4Proto-Germanicfedurþôfourth
Every word from Proto-Germanic fedurþô →Every word from Proto-West Germanic feurþō →Every word from Old English fēorþa →