You can't use the Master theorem on that function $T$.
However, as Raphael suggests, you could consider the related function
$$T'(n) = T'(n/4) + f(n),$$
use the Master theorem to find a solution for $T'$, and then check whether that's a valid solution for $T$ too. No guarantees that it will be, but you could check.
In other words, you could use the guess-and-check strategy to solve the recurrence for $T$, where your "guess" comes from solving $T'$ using the Master theorem. See also Solving or approximating recurrence relations for sequences of numbers for an explanation of guess-and-check (also called guess-and-prove).
One caveat is that guess-and-check will probably require an explicit solution to $T$, with specific constants. In other words, it's usually not enough to guess that $T(n) = O(g(n))$; you will typically need to guess a specific constant $c$ such that $T(n) \le c \cdot g(n)$, one that will enable the proof to go through.