Output impedance of Op Amp with Voltage Divider?
The output impedance at the opamps output node is Zout=Zo/(1+LG) with Zo=open-loop impedance and loop gain LG=(k x Aol). The feedback factor is k=(Rs||R1)/[(Rs||R1)+Rf] and Aol=open-loop gain.

Therefore, the output impedance at the output node is Z6=R4||(R3+Zout). 

Note that Zout in most cases can be neglected.