Op-Amp gain calculation
Here is an alternative solution:

Applying the star-triangle-transformation we get thee other resistors.

However, two of them play no role for the closed-loop gain because they are located at the output to ground (pure load) resp. between the inverting input and ground (no influence on closed loop gain).
Hence, there is only one resistor RF left between output and inverting input (one single feedback resistor).

Using the known resistor values it is no problem to find the value of this feedback resistor RF which is a function of the unknown resistor R4. 

My result: R4=23.995k-0.09998k=23.895kohms.