This is regex here /.(?=.{4})/

Any character followed by any character four times.

function maskify(cc) {
  return cc.replace(/.(?=.{4})/g, "#");
}

console.log(maskify("Awesome"));

If any character is followed by four characters except newline then it will replace it with #

a is followed by weso, replace a with #

w is followed by esom, replace w with #

e is followed by some, replace e with #

After e there is no character which is follwed by 4 characters, so it won't replace it with #