Since $PV = nRT$, the Pressure-Temperature graph at constant volume has a greater slope at lower volume.

Since pt. 2 lies on the line with lesser slope, $V_2 > V_3$.

Since pt. 2 and pt. 1 are on the same line and pts. 3 and 4 are on the same line, $V_1 = V_2$ and $V_3 = V_4$ must hold true, since the P-T graph is a linear one.