Solving the recurrence T(n) = 4T(n/4) + n log n with the iterative method
Your "General Formula" is incorrect.  The third term (with the sum) is incorrect.  It should be $n$ rather than $n/4^k$.

Make the suitable correction through the rest of your answer, and use the fact that

$$\sum_{k=0}^{\log_4 n} n \log_{10}(n/4^k) \le \sum_{k=0}^{\log_4 n} n \log_{10} n$$

and you should be able to get an upper bound from there.  (The final sum is easy to evaluate as every term is the same.)