How do I reset the setInterval timer?

If by "restart", you mean to start a new 4 second interval at this moment, then you must stop and restart the timer.

function myFn() {console.log('idle');}

var myTimer = setInterval(myFn, 4000);

// Then, later at some future time, 
// to restart a new 4 second interval starting at this exact moment in time
clearInterval(myTimer);
myTimer = setInterval(myFn, 4000);

---

You could also use a little timer object that offers a reset feature:

function Timer(fn, t) {
    var timerObj = setInterval(fn, t);

    this.stop = function() {
        if (timerObj) {
            clearInterval(timerObj);
            timerObj = null;
        }
        return this;
    }

    // start timer using current settings (if it's not already running)
    this.start = function() {
        if (!timerObj) {
            this.stop();
            timerObj = setInterval(fn, t);
        }
        return this;
    }

    // start with new or original interval, stop current interval
    this.reset = function(newT = t) {
        t = newT;
        return this.stop().start();
    }
}

Usage:

var timer = new Timer(function() {
    // your function here
}, 5000);


// switch interval to 10 seconds
timer.reset(10000);

// stop the timer
timer.stop();

// start the timer
timer.start();

Working demo: https://jsfiddle.net/jfriend00/t17vz506/

Once you clear the interval using clearInterval you could setInterval once again. And to avoid repeating the callback externalize it as a separate function:

var ticker = function() {
    console.log('idle');
};

then:

var myTimer = window.setInterval(ticker, 4000);

then when you decide to restart:

window.clearInterval(myTimer);
myTimer = window.setInterval(ticker, 4000);