Here are the sentences completed with the correct forms of the adjectives:

1. The **higher** the cost of living in the city, the **more** people need to earn to afford it.
2. The **more modern** the buildings, the **less** character the historic city center might have.
3. The **more** people that live in the city, the **greater** the demand for public services.
4. The **more** green spaces available, the **better** the quality of life in the city.