Planned obsolescence was the first issue I thought of. I don't remember if they specified if only certain replicants would die after 4(?) years, but how would a replicant rise to become one of the best blade runners in such a short amount of time? Does future LA not do background checks? I would think that they would do the test on anyone applying for such a position. Or, they knew all along he was a replicant and that's what they use as Blade Runners. Kind of conspiracy-theoryish, but that could explain it.

Deckard is also considerably weaker than the other replicants in the film. If he was a replicant, why was he so easily outmatched