I take it by your name that you are male, now taking into account your height, weight and activity level you are severely under eating, when you do that you lose weight but some of that weight loss is muscle loss too which is why your abs are less visible, you are simply not eating enough and in order to gain energy your body is eating away at it's own muscles.

You cannot build muscle whilst in a calorie deficit but you can strengthen what you already have, sadly by eating too little what you already have is becoming less and less.

Eat more, I am a recovering anorexic and I have seen so many dietitians so I do know what I am talking about here, whenever I lost weight by under eating my muscles just wasted away, I am almost back at a healthy BMI now but I have no muscle whatsoever, even my arms look scrawny and wasted, you are wasting yours away.

You must never be in more than a 500 calorie deficit per day, cardio 4 times a week makes you at least moderately active so your weight loss intake needs to be at least 2200 and you would still lose 1 pound per week, throw in some strength exercise and you will tone your abs back up too but you must eat enough or there wont be enough left to tone.

A good article below by Mark Sisson.

Washboard Abs on a High-Fat Diet, No Ab Workouts and No Cardio?:

https://www.marksdailyapple.com/high-fat-diet-no-c...