Ideal opamp closed loop problem
Hints: -

why i cant an Req=R2||R3+R4 in feedback circuit.

Your formula finds the Thevenin equivalent resistance but looking into the R4 resistor from the op-amp output node. That will be of some use in your derivation.
You also need to recognize that the Thevenin equivalent voltage at the junction of R3 and R4 is a fraction of that at the op-amp's output. So, find Vx in terms of Vout and then recognize that Vx feeds via R2 creating the negative feedback.
Or regard the op-amp output as being Vx and derive the op-amp gain to Vx (R2 as feedback) then, when you have that formula, convert it to the real op-amp output using the reverse of the attenuation made by R4 and R3||R2.
Then you are good to go.