logics programing

input nums                        =>6,5,4,8

sorted nums                      =>4,5,6,8
indexes of sorted nums   =>0,1,2,3

Clone input nums to sorted nums array thean sort sorted nums, here input nums first number is 6, in sorted nums you have to find index of 6 that is 2. Next input nums is 5, find index of 5 in sorted nums that is 1, Next inpit nums is 4, find index of 4 in 
sorted nums that is 0, Next input nums is 8, find index of 8 in sorted nums that is 3 so output is 2,1,0,3

same process need will be applied for any input nums example, 7,7,7,7

int[] num = new int[] { 6, 5, 4, 8 };
//int[] num = new int[] { 7, 7, 7, 7 };
int[] sorted = (int[])num.Clone();                 
Array.Sort(sorted);
int[] output = new int[4];
int idx = 0;                       
foreach (int item in num)           
{
    output[idx] = Array.IndexOf(sorted, item);
    Console.Write(output[idx] + ",") ;
    idx++;
}

Note : try to improve this code once you understand logic,

All the best for your next interview in advance.