Order of circuit decomposition to find thevenin circuit
You can't consider R3 and R4 in series because of the terminal there on the node between R3 and R4.  In order to be in series, the components have to have the same current path.  With no load connected between the terminals, they would be in series, but that rather defeats the purpose of a source.  So the correct Req would be ((R1 // R2) + R3) // R4.