What's the best way to gain fat while still exercising, i'm currently 4% body fat, and i have a small stomach as in i eat small portions.

Add muscle not fat.: Muscle is way healthier than fat. Fat releases an assortment of bad chemicals into your body. Chemicals that can cause alot of inflammation, with all of the aches and pains that go with it, as well as causing diabetes. Continue eating multiple times & increase your weight through resistance circuit training to build lean, healthy muscle; which will increase your cardiovascular health also.