hilo
The River Nile has only four letters.
There are only three: Iowa, Ohio, and Utah.
agar
duck
In the English language, it is four.
I don't know if it's the only number but the number 4 has four letters in it's name.
cart
john Another answer: While "John" is a name, there is no word that ends in "hn" with only four letters.
Ohio, Iowa, and Utah.
The dodo bird.
Down, Cork and Mayo.
There are truly only 10 countries with four letters and they are Peru, Cuba, Laos, Chad, Togo, Mali, Iran, Iraq, Oman, Fiji, and Guam