ActionScript 3.0 :: Adding Event Listeners To MovieClips - Arrays And For Loops

I'd like to add event listeners to movieclips (already on the stage), whose names are called from an array one by one, into a for loop.

Code:
// there are 4 movielcips on the stage with name a1 - a4.
// create array
var Bro:Array = ["a1","a2","a3","a4"];
// for loop
for ( var i:Number = 0; i < 4; i++) {
// Add event listener to array values to buttons.
Bro[i].addEventListener(MouseEvent.ROLL_OVER, roll, false, 0, true);
//execute this function on rollover
function roll() {
trace("why wont my arrays work in for loops");
}}