var test = [[1,2,[3]],4,[5,[6],[[7,8],9]],10]

var superFlatten = function(arr) {
  return arr.reduce(function(acc, curr) {
    // If item in array is an array, superFlatten(it) then concat, else just concat
    return (Array.isArray(curr)) ? acc.concat(superFlatten(curr)) : acc.concat(curr)
  }, [])
}

console.log(superFlatten(test))