Clean water & salt will do wonders. However, if your hospital tank is small & not cycled, you will need to do lots of water changes to keep the water quality pristine. With a 4 incher, if your tank is less than 10 gal, I would be doing daily 50% changes in the hospital. <I normally use a 30 gal tub as my hospital to minimize the need for pwc.> At any rate, it is good to check the water parameters daily to make sure there is no build up of ammonia & other toxins.
If the wound is not severe, it may be easier to treat the fish in your main tank (since there is no contagious diseases to contend with.) If the wound is looking good, you might just want to return the fish to the main tank after a few days.
BTW, to prevent future mishaps, you might want to cover the filter intake with a guard. (A plastic basket that you can get at the lfs.)