Balanced AC Bridge Solution
In general, the bridge is balanced when Z1/Z3 = Z2/Z4. The frequency and the impedance AC are irrelevant at balance.

Rearranging the expression gives Z4 = Z2*Z3/Z1. I would recommend you only substitute component values after you have algebraically arrived at the final expression for Z4. In general when you multiply the complex quantities Zn = Rn||Cn together, you will find there are terms like (Rn + iwCn) in the denominator. Multiply both numerator and denominator by (Rn - iwCn), recognizing that i*i = -1, which will clear out any imaginary quantities in the denominator, leaving only an expression like (Pr - iPc) in the numerator (Pr and Pc will be complicated expressions). The Pc will correspond to C4, the Pr to R4.