four-day

The compound adjective "four-day" is a modern English formation following the productive pattern of number-unit compounds. The first element, "four," descends from the Proto-Indo-European root kʷetwóres, which gave rise to the numeral across many Indo-European languages, signifying the specific quantity. The second element, "day," originates from the Proto-Germanic dagaz, referring to the 24-hour period of daylight and darkness. Combined with a hyphen, these two native English morphemes create a transparent, attributive adjective that precisely denotes a duration spanning four consecutive days, illustrating the analytic and combinatory nature of English vocabulary building.

Structure: four (number) + day (unit of time) - four: From Old English fēower, of Proto-Germanic origin; related to Dutch vier and German vier, from an Indo-European root shared by Latin quattuor and Greek tessares. Functions as a numerical quantifier. - day: From Old English dæg, of Germanic origin; related to Dutch dag and German Tag. Functions as a noun denoting a unit of time.