In the state of Louisiana, I think they changed the law so they can reduce the population. And save money. So now the offenders get good time and 25% off of their sentence non-violent offenders if so how much time will they do on a 4 yrs sentence

If what you are saying is accurate, 25% off 48 months is 12 months leaving 36 months of time. BUT, there are some other factors that could make the sentence even less, like if the judge allows this convict to have parole. If eligible they could conceivably get more time off but spend it on the streets as a parolee.