T-Feedback Circuit Vo/Vi (Why is my approach wrong)?
Is this correct?


No, because Vs must include R2 as a factor i.e. 

\$V_s = V_o\cdot \dfrac{R_2 || R_4}{R_2 || R_4 + R_3}\$

Where R2||R4 is the parallel resistance of those resistors.


  Is this correct?


No because voltage Vi appears exclusively across R1 and hence sets the current into R2. This is because Vin- is a virtual earth.