Regex to username that contains alphanumeric characters and at most one underscore symbol in javascript

I am trying to validate a username. I want to have a regex that,

1. At least 4 characters long.
2. Must contain only letters, numbers and an optional underscore
3. It must not end with an underscore

i have tried regex,

    /^(?!_*\_*\_*)[A-Za-z]([A-Za-z0-9_]*[A-Za-z0-9])?$/

function validate(username) {
    if ((/^(?!_*\_*\_*)[A-Za-z]([A-Za-z0-9_]*[A-Za-z0-9])?$/).test(username)) {
        return true;
    }
    return false;
}

console.log(validate('hej_gd'));