Maximum Call Stack Size Exceeded During a setTimeout Call

Inside getNum, you’re directly invoking the getNum function, causing the stack to exhaust. Replace the function call getNum() with the function reference getNum:

function getNum() // Gets triggered by page load so innerHTML works
{
    num += 7;     // Increase and assign variable
    document.getElementById('counter').innerHTML = num;
    setTimeout(getNum, 4000);   // <-- The correct way
}

Link to documentation of setTimeout.