code: function encode4(n) {
    assert(0 <= n && n <= 0xffffff, ENCODE_OUTOFBOUNDS + n)
    // 24 bits -> 4 digits
    const lower = isolate(n, 0, 6)
    const aboveLower = isolate(n, 6, 6)
    const belowUpper = isolate(n, 12, 6)
    const upper = isolate(n, 18, 6)
    return encode(upper) + encode(belowUpper)
      + encode(aboveLower) + encode(lower)
  }
docs: Encodes a 24 bit number into four URL safe chars
  
  @name encode4
  
  @param {Number} n a 24 bit number
  @returns {String} the chars