Is it a volitan lion or one of the dwarf or medium bodied lions. Volitans take better to feeding dead food. Sometimes the others may decide not to eat dead food. If it's a volitan does he seem healthy otherwise, how does his color look, has his activity changed at all. Live food like ghost shrimp usually triggers a feeding response, if not then there's some other underlying issue. Four days for a lion really isn't that long, how often do you usually feed him. Every other day to every third day until you see a slight bulge in his belly is better than every day. In the wild they would gorge and fast.
How high is high nitrates