It is very important whether the unit was bypassed or not for that 4 year period. If it was, it should be very close to how it was four years ago. I would run a cap full of bleach into the brine tank to help disinfect it. Then run it through a full backwash. If your showers begin to be slippery in a week or so, your good to go.
If it wasn't bypassed, you may have turned the media into a rock.