four-month

The compound adjective "four-month" is a transparent combination of two ancient Germanic roots. The first element, "four," descends from the Proto-Indo-European numeral kʷetwóres, which stabilized in Proto-Germanic as fedwōr and became fēower in Old English. The second element, "month," originates from the Proto-Germanic mēnōþs, a derivative of mēnô (moon), reflecting the lunar cycle as the original basis for measuring this calendrical period. In modern English, these two morphemes combine directly via hyphenation to form a single lexical unit that precisely specifies a duration, following the productive pattern for creating compound numerical-time adjectives like "two-week" or "five-year."

Structure: four (number) + month (time period) - four: From Old English fēower, from Proto-Germanic fedwōr, from Proto-Indo-European kʷetwóres. Functions as a numerical quantifier. - month: From Old English mōnaþ, from Proto-Germanic mēnōþs, related to mēnô ("moon"). Functions as a noun denoting a unit of time.