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
Ohio, Iowa, and Utah.
The dodo bird.
Down, Cork and Mayo.
john Another answer: While "John" is a name, there is no word that ends in "hn" with only four letters.
There are truly only 10 countries with four letters and they are Peru, Cuba, Laos, Chad, Togo, Mali, Iran, Iraq, Oman, Fiji, and Guam