code: function decode4(s) {
    assert.equal(s.length, 4)
    const [ upper, belowUpper, aboveLower, lower ] = s
    return (decode(upper) << 18) | (decode(belowUpper) << 12)
      | (decode(aboveLower) << 6) | decode(lower)
  }
docs: Decodes four chars into an 24 bit number
  
  @name decode4
  
  @param {String} s the chars to decode
  @returns {Number} a 24 bit number