Half vs. Full-Bridge
A full bridge setup will let you have an output voltage range going from  v3 - v4 (2 volts here) to -(v3-v4) (-2 volts here)

A half bridge setup will only let you have half that range, i.e. from 1 to -1 volts

This is because is a full bridge, as opposed to a half bridge, the output voltage is not referenced to ground, but rather one of v3 or v4.