Maybe match on a simple regex to get each group of numbers, then map over that array to produce the desired result, joining it up into a new string at the end.

const str = '1234 1234 1234 5678';
const re = /(\d+)/g;

const out = str
  .match(re)
  .map((el, i, arr) => {
    if (i < arr.length - 1) return '****';
    return el;
  })
  .join(' ');
  
console.log(out);

You can simply slice the string and save its four-last characters and append it to the result string after your done with the replacement

let str = '6135 4639 4990 1032'

function hideChars(str) {
    const stars = str.slice(0, str.length - 4).replace(/\d/g, "*")
    const fourLast = str.slice(str.length - 4, str.length)
    return stars + fourLast 
}
const result = hideChars(str)
// result = **** **** **** 1032