my brother got 4 years in prison he did 8 months in county jail how much more time would he need to do?

If the sentence is 48 months, with good time credits he will do 85% of the sentence or 40.8 months. The eight he's already done leaves him with 32.8 months remaining, provided he behaves and does not lose the 15% they get when they arrive.

There is always the question of parole. If this is a state charge and the judge wrote "parole" in the Judgment and Commitment Order, then he might get a Parole Board Hearing at 1/3 of the served time (this would come at the 8 month mark).