addEventlistener, array, as linkage

I have added 10 movieclips with the AS linkage "Box" in this function. I've named the different instances "layer1" to "layer10"

My question is, how can one add an eventlistener to, lets say, only "layer4"?

var NUM_BOXES:int = 10;

var BOX_SPACING:int = 1;

var _boxes:Array = [];

function Test()

                    {

                              for (var i:int = 0; i < NUM_BOXES; i++)

                              {

                                        var box:Box = new Box( i + 1 );

                                        box.y = (box.height + BOX_SPACING) * i;

                                        box.name= "layer" +( i + 1);

                                        box.buttonMode = true;

                                        box.addEventListener( MouseEvent.MOUSE_DOWN, onBoxPress );

                                        box.addEventListener( MouseEvent.MOUSE_UP, onBoxRelease );

                                        addChild( box );

                                        _boxes.push( box );

                              }