He can do the whole 4, although it is unlikely. I have seen some cases where those who got low sentences did get "serve all"s to save them from having to supervise on parole.
Potentially he can do the whole four years. He will be eligible for parole after just a few months. If he does not make parole the first time, he will continue to have periodic parole hearings. Having an attorney represent him at his hearing may improve his chances.
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