4 tablespoons per gallon to me is an excessive dose. That is roughly 60 ml by volume which will be over 60 grams per 4 liters. 1 Tablespoon = 15 ml
I use Epsom salts at 1 teaspoon (5ml) per gallon (3.8 liters). I use it once every 2 or 3 months, or if I see any signs of chlorosis. Leaves turning yellow with the viens keeping a green color is classic chlorosis, and time for a dose of Epsom salts, magnesium sulfate. I usually do a dose of an iron supplement at the same time.