Op-amp voltage divider
V+ is a voltage divider of V1 (V+=V1/2) because the OpAmp draws (virtually) no current on its input pins. And no, R4 and R2 are not in parallel. Even if the voltage across them is the same (thanks to the OpAmp enforcing a virtual short), they are not sharing the same nodes!

Once you know V+, the rest is easy: 


If you are used to these circuits, you may see the circuit as a non-inverting amplifier of V+.
You can compute the current through R4, notice that the current through R3 is the same and compute the output Va accordingly -rebuilding the non-inverting amplifier theory.