Nexus-6 have a built-in four-year lifespan according to Wikipedia
http://en.wikipedia.org/wiki/Nexus_6
So, Tyrell added a "fail-safe device" to Nexus-6 models: a built-in four-year lifespan to prevent them from developing their own "emotional responses" and emergent independence. This was especially necessary for Mental-A models whose intellectual capacity at least matched those of their designers.