The math works in his favor and the eight months in county is already money in the bank.
A four-year sentence is 48 months total. With the standard 85% good time calculation applied, he needs to serve about 40.8 months assuming a clean disciplinary record throughout. Subtract the eight months already served in county and he has roughly 32.8 months remaining from this point forward, just under two years and nine months.
That calculation assumes two things. First, that the eight months in county is being credited toward the state sentence, which it should be in virtually every jurisdiction. Second, that he protects his good time by avoiding incident reports and disciplinary infractions going forward. Every violation chips away at that 15% credit and adds time back onto the release date.
The other variable worth checking is parole. If this is a state charge and the sentencing judge included a parole provision in the Judgment and Commitment Order, your brother may be eligible for a parole board hearing at around the one-third mark of his served time. That hearing could come considerably earlier than the 40.8-month calculation suggests. An attorney or his case manager can pull the commitment order and confirm whether parole is written into the sentence.
If parole is available and he maintains a strong institutional record, his actual time served could be significantly less than the 32.8 months remaining on the good time calculation. That is the best-case scenario and it is worth pursuing actively from day one.