The exact mechanism for limiting lifespan is never revealed in the film. However, it would not seem to be something as "simple" as telomere length since that would lead to some variability in lifespan and presumably signs of aging (e.g., like <PERSON>). However, not only does <PERSON> die looking exactly as he did at incept, when he dies he really just seems to "switch off." Certainly, it wasn't as abrupt as a switch. He obviously knew it was coming, as the film made clear by his use of a nail through the hand to delay onset of death. But it was only a matter of minutes from the earliest such signs conveyed by the film until his death.
EDIT: I've rearranged my answer to place the answer to the OP's question at the top. The remainder of my answer remains below and serve less to answer the OP's question, and more to rebut other answers/comments.
Regarding motivation, the movie itself answers this question directly:
<PERSON>: Well I don't get it. What do they risk coming back to earth for? That's unusual. Why--what do they want out of the Tyrell Corporation?
<PERSON>: Well you tell me pal, that's what you're here for.
<PERSON>: [funny look]. [pause] What's this?
<PERSON>: Nexus 6. <PERSON>. Incept date 2016. Combat model. Optimum self-sufficiency. 865
Probably the leader. This is <PERSON>. She's trained for an off-world kick-murder squad. Talk about beauty and the beast, she's both. The fourth skin job is Pris. A basic pleasure model. The standard item for military clubs in the outer colonies. They were designed to copy human beings in every way except their emotions. The designers reckoned that after a few years they might develop their own emotional responses. You know, hate, love, fear, anger, envy. So they built in a fail-safe device.
<PERSON>: Which is what?
<PERSON>: Four year life span.
That explanation in the film is straightforward and to set it aside means you have to disbelieve <PERSON>. Leaving aside his obvious "charm", I see no evidence in the film that <PERSON> shouldn't be trusted on this seemingly benign bit of background information established early in the film, particularly where he's never established as being an unreliable narrator in other ways.
I take it that <PERSON>'s proclamation that "[t]o make an alteration in the evolvement of an organic life system is fatal" and "[a] coding sequence cannot be revised once it's been established" is intended to convey the idea that <PERSON> could have given them some other lifespan, but cannot do so after the fact. Though not established as fact per se, I believe <PERSON> is a replicant which if true also establishes that 4 year life span is not an artifact of being a replicant (due to, e.g., a lack of "cell replacement"), but a design choice. 861