It's not possible to make a proper flex box / flex wrap polyfill, but this one solves one particular problem.

flexFix.js

(function(global){
    'use strict';
    function flexFix() {
        var list = document.querySelector('.flexFix');
        if (!list) { return ;}
        var items = list.children;
        if (!items.length) { return ;}
        var newContainer;
        var perLine = 4;
        var stopper = items.length % perLine + 1;
        while (items.length > stopper) {
            if(!newContainer || newContainer.children.length === perLine) {
                newContainer = document.createElement(list.tagName);
                newContainer.className = list.className;
                list.parentNode.insertBefore(newContainer, list);
            }
            newContainer.appendChild(items[0]);
        }
    }

    /* global Modernizr */
    if (Modernizr && Modernizr.flexbox === false) {
        global.addEventListener('DOMContentLoaded', flexFix, false);
    }
})(this);