How do I handle big integers in C#?

I have a function that will give me the product of divisors:

private static int GetDivisorProduct(int N, int product)
    {
        for (int i = 1; i < N; i++)
        {
            if (N % i == 0)
            {
                Console.WriteLine(i.ToString());
                product *= i;
            }
        }

        return product;
    }

The calling function is GetDivisorProduct(N, 1)

If the result is bigger than 4 digits , I should obtain only the last 4 digits. ( E.g. If I give an input of 957, the output is 7493 after trimming out only the last four values. The actual result is 876467493.).

Other sample inputs: If I give 10000, the output is 0.

The BigInteger class has been removed from the C# library!

How can I get the last four digits?