var rand_pool = ["a","b","c","d","e","f","g","h","i","j","k","m","n","p","q","r","s","t","u","v","w","x","y","z","1","2","3","4","5","6","7","8","9"];
var rand_word = function(){ return rand_pool[parseInt( rand_pool.length * Math.random() )]};
var rand_four = rand_word() + rand_word() + rand_word() + rand_word();
console.log(rand_four);