28yrs. Old and 4 kids later. I'm trying to lose the big belly on my small frame. 100 situ po s and or either 100cruches. Help not working. Any advice?

Be patient. The abdominal muscles were stretched into a different length for your several pregnancies over several years. It can take a couple years of doing regular crunches to undo that stretch, but it can. If your belly button is sunken in more than half an inch, you also carry chub that you lose with dietary changes, not exercise. It is certainly worth your effort and easy to understand the frustration.
Add aerobic exercise. Situps and crunches will tone and strengthen your abdominal musculature but do not burn many calories; regular aerobic exercise combined with a strength training program is your best bet at ramping up your metabolism and burning off belly fat. If you have not been active for a while or have other medical issues speak to your physician about what types of exercise would be best for you. Good luck.