It appears as though the leading cause of death in replicants is a form of cellular death, akin to 'old age' in humans.

Remember that in the first movie, Tyrell admits that the short lifespan was not by design, but was the only way they could create them in the first place when he says:

> “You were made as well as we could make you.”

Much like humans, anything can reduce this life-span; not to mention the lack of accuracy in any lifespan. They are not robots, remember, they are still 'organic', despite their synthetic nature. As humans have a  natural lifespan of 80 yrs (+/-20), it would appear Replicants are 4 years (+/-1).