In that short period of time, you likely made the outer part of the ham less salty, but the salt more towards the middle didn't have time to equalize. On something of that size, I'd suggest about a 3 day equalization period to give the salt time to move from areas of high concentration to areas of low concentration.
As for changing the water, I'd do it once a day, or maybe once every 12. Water can hold a stunning amount of salt. You can dissolve right at 3 pounds (!) of salt in one gallon of water. Obviously you want the water less salty than the ham to get the salt to move out, but I don't think you'll lose much of the osmotic gradient in 4 hours.