Why doesn Deckard know about the replicants four-year lifespan?
While the film never states one way or the other, it can be inferred that the four-year lifespan was a new development, specific to the Nexus-6 model; otherwise, it would seem to make no sense that Deckard, an experienced blade runner, would have to be told this fact by Bryant. Since we know that Deckard had been retired from the job for an undisclosed amount of time, he might not be familiar with the newer model replicants, and required a catch-up briefing.
