It is not enough to just do crunches. You have to do cardio as well to get rid of most of your fat. You don’t want to only do core exercises so if you want a 4 pack, you need to commit to a more well-rounded exercise routine. You’ll want to work your arms, legs, back, shoulders, etc. But to specifically build abs, you’ll want a cardio routine – not walk a couple miles on the treadmill, but run several miles several times a week until you are dripping in sweat and are exhausted. You can work in other types of cardio to keep things interesting like swimming (try doing butterfly stroke), paddling (not sure about rowing but in outrigger paddling you get some workout in your obliques), etc. You’ll want to supplement the cardio with weight training. Do sit-ups, do sit-ups on a decline bench, do sit-ups on a decline bench with weight. Don’t just sit-up. Do sit-ups to the sides as well (basically it is a sit-up until the end where you twist your body to finish to the right or left of your feet; you’ll want to get better instructions than these, btw, for safety reasons pls don’t rely on this text alone) to work your obliques. There are other ab workouts you can do besides sit-ups that will enhance your workout. The second part that complements the exercise is your diet. You can’t be munching bacon all day everyday and expect your workout routine to get you a 4/6/8-pack. You need to be burning more calories than you intake. You might be able to do this by feel after a while, but if this is an issue for you I would take steps to ensure if you’re serious about your goal. And no, vitamin supplements are not necessary, but it depends what you’re eating.