[AndreyMG]

В голову пришел только такой дурацкий способ:

var op = {name: name, address: "0000"};

var number = m_Operands.length;
var address = number.toString(16).toLowerCase()  ;

op.address = op.address.split("");

op.address[3] = address[0] != undefined? address[0].toString() : "0";
op.address[2] = address[1] != undefined? address[1].toString() : "0";
op.address[1] = address[2] != undefined? address[2].toString() : "0";
op.address[0] = address[3] != undefined? address[3].toString() : "0";

op.address = op.address[0].concat(op.address[1], op.address[2], op.address[3]);

Если возникнут идее лучше - отпишитесь пожалуйста!