Firstly, you shouldn't eat between meals. You should be eating normally and with a healthy diet (ie. morning cereal, salad/fruit lunch, red meat or fish dinner). Fruit snacks in between are good in between meals. Energy bars are also good for your cause.
Secondly, 20 skips and 20 sit-ups is only a start to losing weight. Gradually you'll have to increase that number to 40, 60, 80, 100, 120, etc etc. Also I don't know what you mean riding an exercise bike to the point of exhaustion means. If you ride very hard for 10 minutes and you're "exhausted" that doesn't help either. You should be on the bike at least 30 minutes to start out with.
Thirdly, you can't really expect to dramatically lose a lot of weight in 4 days time. Losing weight takes weeks, even months to complete. You're body is still adjusting to your new program, and once it's full adjusted, you will feel more conditioned and stronger, and be able to exercise longer. You just need to be patient and continue with the same routine, if not a little more. Gradually you'll be able to see results.