hilo
The River Nile has only four letters.
agar
Fiji (Suva) Peru (Lima) Togo (Lome)
There are only three: Iowa, Ohio, and Utah.
The largest country in the world is Russia and has more than four letters, but the largest continent in the world is Asia with four letters.
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.