I think u did it like Vo*R2/(R1+R2). It's wrong because R2 has in parallel (R3+R4). So, it should be Req=R2||(R3+R4); Vo*Req/(R1+Req).
Then the voltage that it comes out to be say, V1, should be used like: Vth=V1*R4/(R4+R3).
Or, simply use nodal analysis. Assign voltage V1 to 1st node from left and next node has already voltage=Vth.
Write 2 equations corresponding to the 2 nodes with voltages V1, Vth. Solve them using cramer's rule or using calc.
U will get them in a step.