function steamrollArray(arr) {
  // I'm a steamroller, baby
  
  console.log(arr, '->')
  let arrUnchanged = false
  while (!arrUnchanged) {
    let initialLength = arr.length
    for (let i = 0 ; i < initialLength ; i++) {
      if ( Array.isArray(arr[i]) ) arr.splice(i, 1, ...arr[i])
    }
    arrUnchanged = arr.every(elem => !Array.isArray(elem))
  }
  console.log(arr)
  return arr;
}

steamrollArray([1, [2], [3, [[4]]]]);