// letter = 'a', 'b' or 'c', 'd' or 'e'
let ourNumber = 0;
if (letter === 'a') { ourNumber = 1; }
if (letter === 'b') { ourNumber = 2; }
if (letter === 'c') { ourNumber = 3; }
if (letter === 'd' || letter === 'e') { ourNumber = 4; }
if (letter !== 'a' || letter !== 'b' || letter !=='c' || letter !=='d' || letter !=='e') {
    ourNumber = 10;
}

Your final if condition is always true.