wasn't that a new feature of the 'new' nexus 6's...?

I have to agree that it was really heavy handed exposition.

Bryant: (sic) The designers reckoned that the Nexus 6 would develop their own emotional responses, you know, hate, fear, love, envy...*so they built in a fail-safe device*.

Deckard: Which is what?

Bryant: Four year life span.

If the replicants didn't have this, they would eventually be indistinguishable from humans in a Voight-Kampf test. As the Blade Runner, Deckard had to know what the fail-safe device was. It was also the motivation for the Replicant's return to earth....to seek immortality..

Decard: I don't understand. Why would they return to Earth? That's...unusual.
Bryant: You tell me pal, that's what you're here for.