public static int[,] Combine(int[,] mas) {
    if(mas.GetLength(0) < 2) return null;
    int len = mas.GetLength(1), index = 0;
    int[,] result = new int[4, len * --len * len * ++len / 2];
    for(int i = 0; i < len; ++i)
        for(int j = 0; j < len; ++j)
            for(int k = 0; k < len; ++k)
                for(int l = 0; l < len; ++l) {
                    if(i == k || i == l || j == k || j == l)
                        continue;
                    result[0, index] = mas[0, i];
                    result[1, index] = mas[0, j];
                    result[2, index] = mas[1, k];
                    result[3, index] = mas[1, l];
                    ++index;
                }
    return result;
}