What's wrong with my MergeSort? C#

Question

To keep it short. I made a MergeSort, which should sort an int[]. To test the MergeSort I made an array containing 4 integers in random order. It returns the exact list in the order given to the MergeSort, instead of doing what it should do. I wonder what I did wrong.

Edit: The first two merges return these two lists: 4, 15 && 5, 16, however the last merge doesn't work.

This is the actual code:

 public static int[] Merge(int[] list, int left, int mid, int right)
    {
        int[] returnList = new int[list.Length];
        int Index = left;

        //last value of left half
        int leftEnd = mid;

        //both the left side and right side contain atleast one value
        while (leftEnd >= left && right >= (mid + 1))
        {
            if (list[left] <= list[(mid + 1)])
            {
                returnList[Index] = list[left];
                left++;
            }
            else
            {
                returnList[Index] = list[(mid + 1)];
                mid++;
            }
            Index++;
        }

        //right side is empty, left contains atleast one value
        while (leftEnd >= left)
        {
            returnList[Index] = list[left];
            left++;
            Index++;
        }

        //left side is empty, right contains atleast one value
        while (right >= (mid + 1))
        {
            returnList[Index] = list[(mid + 1)];
            mid++;
            Index++;
        }

        return returnList;
    }

    public static int[] Split(int[] list, int left, int right)
    {
        if (right > left)
        {
            int mid = (right + left) / 2;

            Split(list, left, mid); //first half
            Split(list, (mid + 1), right); //second half
            list = Merge(list, left, (mid + 1), right);
        }

        return list;
    }

My Main:

        int[] intArray = new int[] { 12, 4, 16, 5 };
        int[] ReturnTest = MergeSort.Split(intArray, 0, intArray.Length - 1);

        foreach (int value in ReturnTest)
        {
            Console.WriteLine(value);
        }

The expected output: 4, 5, 12, 16

The actual output: 12, 4, 16, 5