function flatten(arr) {

  return [].concat(...arr)

}

function deepFlatten(arr) {

  return flatten(           // return shalowly flattened array

    arr.map(x=>             // with each x in array

      Array.isArray(x)      // is x an array?

        ? deepFlatten(x)    // if yes, return deeply flattened x

        : x                 // if no, return just x

    )

  )

}

​

var fourDimensionalArray = [

  [

    [

      [1, 2],

      [3, 4]

    ],

    [

      [5, 6],

      [7, 8]

    ]

  ],

  [

    [

      [9, 10],

      [11, 12]

    ],

    [

      [13, 14],

      [15, 16]

    ]

  ]

];

​

console.log(deepFlatten(fourDimensionalArray))