Update 04ModelDesign/exercises.txt

![question 4-3](https://f.cloud.github.com/assets/3165157/81533/84e3101a-633a-11e2-921f-6fef41a90bad.jpg)
![question4-4](https://f.cloud.github.com/assets/3165157/81534/96e6b6f4-633a-11e2-9470-48631b4749b5.jpg)