Is a copper pour redundant on the top layer of a multilayer (>= 4) board with a ground plane?
In a multilayer board with a GND plane directly under the top layer,
  does adding a copper pour to the top layer give extra value in terms
  of reducing crosstalk and EMI? If all signal traces already have an
  adjacent GND plane, can a copper pour provide additional benefit?


Can a Faraday cage work just as effectively with one wall open?



The picture shows that devices are protected from an external field in all directions i.e. the more sides of the "cage" you have, the more complete the protection.

It works both ways and not just against high voltages (incoming). Local emissions (outgoing) of EMI are significantly reduced when using a "full cage" and this is what happens when you fill the "gaps" up on a PCB.

Consider also the "patch" antenna: -





It uses a ground plane and the top copper emits EM waves. It relies on the dimensions to the bottom ground plane and the width of the top copper to obtain resonance at just the right frequency. If that top-copper is surrounded by grounding copper then its effect is disrupted greatly.