Transfer function and bode plot of an op-amp circuit
The 4th pin could be one of two things:
A reference pin, similar to those found in instrumentation amps to offset the voltage.

The negative rail of the op amp (which I don't think it is, but could be)

Either solve it for both assumptions, or ask the designer of the question what they intended.

Also, your assumption of 0V for the voltage across the indicator doesn't look right. I would think that the opamp is going to drive some kind of current through the 4th pin.