„The ICAO, NATO, and FAA use modifications of English digits as code words, with 3, 4, 5 and 9 being pronounced tree, fower (rhymes with lower), fife and niner. The digit 3 is specified as tree so that it is not pronounced sri; the long pronunciation of 4 (still found in some English dialects) keeps it somewhat distinct from for; 5 is pronounced with a second "f" because the normal pronunciation with a "v" is easily confused with "fire" (a command to shoot); and 9 has an extra syllable to keep it distinct from the German word nein "no". (Prior to 1956, three and five had been pronounced with the English consonants, but as two syllables.?“

https://en.m.wikipedia.org/wiki/NATO_phonetic_alphabet#Pronu...