Google a tdee calculator. That will give you a better idea of what you need in a day and go from there. I do 20:4 fasting with between 1300-1500 calories. It is VERY hard to get that in within that time frame for be but most days I get it done. I do a fat bomb at the end of my eating window if Im not over 1200
I would give your body enough time before starting 20:4 and low carb at the same time. Perhaps start with one of the two? If you are not fat adapted, getting in those calories in a single meal with a lot of fat can be hard to digest. Its better to use the entire 4 hour window to eat enough. Also make sure you keep track of your micronutrients..