I have never had pain from epsom salts. I always soak in the tub and don't use foot baths.
You may be using too much. I'd cut back to an ounce/gallon, maximum.
4 quarts is not very much water. At high concentrations it might sting like any salt would on any open areas of the skin.
Examine your feet and see if you have cracks in the skin, or athlete's foot, or any open areas. People with numbness in the feet often have small injuries they don't notice.
Now would be the time to find these areas, before they become infected with something. (fungal or bacterial).
__________________
All truths are easy to understand once they are discovered; the point is to discover them.-- Galileo Galilei
************************************
.
Weezie looking at petunias 8.25.2017
****************************
These forums are for mutual support and information sharing only. The forums are not a substitute for medical advice, diagnosis or treatment provided by a qualified health care provider. Always consult your doctor before trying anything you read here.