water softener not used for 4 years
 

Just added salt to my water softener that has not been used for 3-4 years

The water is a little yellowish, is this rust?

Can I fix this?

Any damage done to the softener by not filling it with salt it for 4 years?

Full of bacteria, rust, iron, calcium, arsenic, etc....
Replace it.

if the unit wasn't bypassed is the unit still running with no salt in the tank?

If there was no salt in the tank then the media is most likely unsalvagable.

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.

what do you mean by the media?

The media is the resin that does the actual softening. The particles that the water runs through for ion exchange to take place.