The "Director's Cut" version of the original movie (which I consider the true canon) makes clear that Deckard himself is a replicant, created to take that case.

I must have been watching a different Director's Cut, because that is not what I came away with watching the ending.  I came away with the idea that it was left absolutely ambiguous on whether Deckard was a replicant or a human.  It depended on what you thought it was.

Of course, if they studio had not gone with the voice-overs (Which barring the ending, I liked.), we would not have gotten the 'tears in rain' dialogue that Scott choose to keep in the Director's Cut version.

C.T.

I prefer the theatrical version. Includes the voice-over, no unicorn dreams, Deckard is not a replicant (if he is, the story is utterly ruined for me), Gaff leaves the origami unicorn for no other reason that to let Deckard know he was there and did him a solid, they ride off into the sunset and an uncertain future.

Hey, I'm an enigma.

Also, replicants are built with the 4 year life spans to prevent them from developing their own personalities - and thus start to demand rights, which is undesirable. Once the lifespan is encoded into their DNA when they are produced, it's can't be altered (that's what Tyrell told Roy, and who knows if he was lying or not), but they can be made with any lifespan the designers choose. Rachel's lifespan is not noted in the movie, only that she was special and didn't have any particular limit encoded, which makes sense because the entire point of the experiment was to find out if her emotional programming would give her a longer useful duration (a failure to be sure). Over the years I have always found Rachel's character extremely compelling, and I'm not exactly sure why. Tyrell's niece must have been quite a woman.