var s = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ01234567890';
function f(s, width){
  return s.replace(
  new RegExp('[//s//S]{1,'+(width || 4)+'}', 'g'),
  function(m){
    return '^' + new Array(m.length).join(' ');
  });
}

console.log(s);
console.log(f(s, 4));
console.log(f(s, 8));