I have a monterey jack that also is low on salt. From what I understand, you want about 2.2-2.5% by weight salt. So, if your 4 gallons made 4.5 lbs, you'd want 4.5 lbs x .025 = 0.1125 lbs of salt. I don't know how much that is, but if I convert it to grams, I know I get about 20 grams of salt per tablespoon. So, 0.1125 lbs x 450 grams/lb = 51 grams. So, you would need about 2 1/2 Tbsp, if you also get about 20 grams of salt per tablespoon.
The cheese I made with low salt, though, I think happened because a lot of the salt got washed away in the whey. In the future, I'll be a lot more careful to add the salt slowly, and to try to ensure not too much gets in the whey. Good luck!