Yes, Nika, since apple cider vinegar has no calories. Shall I presume that you are doing a water Fast? If you are eating nothing, the cider might hit your stomach rather hard…. I know it’s all acid, but try a little first to see how it sets.
What are your goals for the Fast? How many days? Will you repeat this every month, as Dr Longo suggests? If your purpose is to lose weight, then a 4-day Fast will change your weight for a little bit, but will it change your over-all eating plan? Food for thought.
Good luck.