Calculating thevenin equivalent for a circuit with resistors in a configuration I don't understand
I would just like to give another way of calculating the desired voltage by finding a Norton equivalent, which in result simplifies the circuit and should give you no trouble. 

Find the current that goes through ab by short-circuiting the R4 resistor (remove it from the circuit and just place a wire instead of it). You have already found the thevenin resistance so you just have to find the current through ab (which has no R4 resistor) and multiply the two. $$I_{ab}R_e=V_T$$ I find this the easiest way of solving this.