Prior to exposure, the photographer had to turn one of them into the focal plane using the metal key (visible in lower left image). Following the exposure, the exposed plate was returned to its position against the side of the camera, and another plate was turned into position. This  procedure was repeated until all four plates had been exposed.

The Quad was advertised as being "...the quickest working camera on the market," and claimed that one could take four shots on its four glass plates in only five seconds! I gave it a try (without plates) and found that it would have been an amazing feat.