You can only allocate your available citizens to hexes, so you can only work four hexes at this time. Besides the fact that developed land connects resources to your empire, the rest of your land is basically lying fallow.
Now, you still have very low production, so I'd say open up the manual citizen management tab in the top-right of this city management pane, and select "production focus". Often, when you annex cities you can have it set to a manual control that you don't want or, when you have small cities, the automatic controls could be defaulting to a food focus instead of a production focus.
However, more people can lead to more production, so you'll have to find a balance between making food to grow your city (to have more production) and making production to build the wonder.