I assume your second pin "1" is supposed to be "3" in your diagram.
So for 240V operation you connect pins 2 and 3 together and connect the 240 volts to pins 1 and 4. That connects the two windings in series.
For 120V operation the two primaries are connected in parallel (which is similar to using half the primary, but reduces the transformer primary resistance).
I assume your second pin "1" is supposed to be "3" in your diagram.
So for 240V operation you connect pins 2 and 3 together and connect the 240 volts to pins 1 and 4. That connects the two windings in series.
For 120V operation the two primaries are connected in parallel (which is similar to using half the primary, but reduces the transformer primary resistance).