It takes longer than 4 days to adjust to a low-carb diet. You’d probably be experiencing the same thing if you were doing Atkins. Actually, Atkins has an intermediary stage that allows some carbs (maybe 50g/day?) for a week or so. I think the protocol should have something similar, since starting the cleanse straight from a regular Western diet is practically suicide. Anyway, as your body adjusts to a fat-based metabolism, you’ll regain your energy as long as you get enough calories through protein and fat sources. Veggies have a small amount too.