[3dartmax]

window.addEventListener("DOMContentLoaded", function() {
    var content = document.querySelector(".slider");
    content.addEventListener("click", function(event) {
        var target = event.target;
        if (target = target.closest(".btn")) {
            event.preventDefault();
            var a = content.querySelectorAll("#redBtn, #yellowBtn, #greenBtn, #blueBtn");
            var img = content.querySelectorAll(".preview");
            var selected = [].indexOf.call(a, target);
            [].forEach.call(a, function(el, i) {
                 (i == selected) ? (el.classList.add("actBtn"), img[i].classList.add("act")):
                  (el.classList.remove("actBtn"), img[i].classList.remove("act"))
            })
        }
    })
});

Верно?