four-digit

The compound adjective "four-digit" is a transparent modern English formation, combining the native English number "four" with the classical loanword "digit." The word "four" has ancient roots, descending from the Proto-Indo-European numeral kʷetwóres, which gave rise to its forms across Germanic (English), Italic (Latin quattuor), and Hellenic (Greek tessares) languages. "Digit" entered English via Middle English from Latin digitus, literally meaning "finger" or "toe," reflecting the universal practice of counting on one's fingers. The logical semantic evolution is direct: a "digit" became a symbol for any of the ten numbers (0-9), and "four-digit" precisely describes any numerical value represented by four such symbols, such as a year or a PIN code.

Structure: four (number) + digit (numeral) - 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. - digit: From Latin digitus meaning "finger, toe". In English, it evolved to mean a numeral from 0 to 9 because counting was originally done on the fingers. Functions as the entity being counted.