This code is supposed to reject pin codes that aren't 4 digits or 6, and its supposed to reject codes that have letters in them

The code rejects pins with letters which it's supposed to do, but it rejects valid pins what did I do wrong

function validatePIN (pin) {
let letters = /^[A-Za-z]+$/;
if (pin.match(letters)) {
return false;
if (pin.length = 4 || 6) 
return true;
} 
return false;
}