const out = [];
function steamrollArray(arr) {
  for (let item of arr) {
    if (Array.isArray(item) === false) {
      out.push(item);
    } else if (Array.isArray(item)) {
      steamrollArray(item);
    }
  }
  return out;
}

console.log(steamrollArray([1, [2], [3, [[4]]]]));