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/