easy
A harp has four of something, whilst a guitar has six of them. What are we talking about?
Common Wrong Answers
“Strings”
While a harp does have strings and the number can vary, the riddle is specifically asking about the number of letters in the words 'harp' and 'guitar', not the actual strings themselves.
“Chords”
Chords are musical elements that can be played on instruments, but the riddle is focused on the number of letters in the words rather than the musical components of the instruments.
“Frets”
Frets are physical features on stringed instruments, but they do not relate to the number of letters in the names of the instruments, which is the core of the riddle.