The energy released by an explosion is the motor of the extension of its blast. For a time this energy is faced by the density of the violently displaced ambient medium. During this regime the edge moves according to Taylor's 2/5 scaling. However the explosion eventually departs from this trend. In this forth episode on the physics of explosions we show that this departure is due to the growing influence of the ambient pressure rather than density. Ultimately, this pressure combines with the energy to produce a final blast radius, beyond which the shock front becomes increasingly indistinguishable with a simple sound wave.