Finding recurrence when Master Theorem fails
OK, try Akra-Bazzi (even if Raphael thinks it doesn't apply...)
$$
T(n) = 4 T(n / 2) + n^2 / \lg n
$$
We have $g(n) = n^2 / \ln n = O(n^2)$, check. We have that there is a single $a_1 = 4$, $b_1 = 1 / 2$, which checks out. Assuming that the $n / 2$ is really $\lfloor n / 2 \rfloor$ and/or $\lceil n / 2 \rceil$, the implied $h_i(n)$ also check out. So we need:
$$
a_1 b_1^p = 4 \cdot (1 / 2)^p = 1
$$
Thus $p = 2$, and:
$$
T(n) 
  = \Theta\left(n^2 \left( 1 + \int_2^n \frac{u^2 du}{u^3 \ln u} \right) \right)
  = \Theta\left(n^2 \left( 1 + \int_2^n \frac{du}{u \ln u} \right) \right) 
  = \Theta(n^2 \ln \ln n)
$$
(The integral as given with lower limit 1 diverges, but the lower limit should be the $n_0$ for which the recurrence starts being valid, the difference will usually just be a constant, so using 1 or $n_0$ won't make a difference; check the original paper.)

[I've taken the liberty to add this to the Akra-Bazzi examples in the reference question, thanks!]