Simplifying circuit (Theveninøs theorem)
To find the Thevenin equivalent resistance (Rth) we need to...


Replace voltage sources with short circuits
Replace current sources with open circuits
Calculate the resistance as it appears from the terminal.


For your case, short circuit the voltage source. This will eliminate R4 as current will take the path of least resistance and now that you have a s/c across R4, current will flow through that.

So we redraw the schematic with this new information...







simulate this circuit &ndash; Schematic created using CircuitLab

Now looking at the circuit, the observed resistance between point A and B can be calculated...

Rth = (R1 + R2) || R3


That is, Rth is R3 in parallel with R1 and R2 in series. You should remember how to calculate resistors in parallel.

Now to determine what the equivalent voltage source would be, lets look at the circuit in a different light...







simulate this circuit

All we've done here is move some components around to make it easier to understand. As you'll see, the voltage drop across R4 is the same as the voltage drop across R1,2,3 so we can ignore it as it doesn't help us determine Vth.

Using the voltage divider rule, we can figure out Vth as the voltage drop across R3. In this case...

Vth = V * (R3 / (R1 + R2 + R3))


That is, R3 will have a voltage drop that is proportional to the resistance is adds to the branch compared to the total resistance of the branch. Conveniently, if all values for R1, R2 and R3 are the same...

Vth = V / 3


This is because 1/3 of the voltage drop will occur across R3.

Finally, redraw the circuit to have a voltage source of Vth and a series resistor of Rth.