3 Answers

If you are saying the sentence was for a period of 4 years, but the actual time incarcerated is going to be 2 years, I'm on the same page.  If you consider 365 days to be one year, then 730 days would be two years.  Having already served 500 days, one would think only 230 days remained (about 8 more months).  HOWEVER, with all this "new math" in calculating time served, it may be considerably less.  Some facilities give credit for "good behavior". Some count 3 days for every 2 days served. 
I can't really tell you an answer you can put money on because of the variables. 

a year

That all depends. Maybe 6 months. But get caught with a weapon in the prison , you will get 4 more years, sexual assault, 6 more years. Stay clean .