4. Heather earns $\$ 0.02$ for every aluminum can that she collects. In this situation, what does the independent variable represent? the amount of money she earns $\$ 0.02$ The number of cans she collects.

Step 1 ：The independent variable in this situation is the factor that is manipulated or changed and affects the dependent variable.

Step 2 ：In this case, the number of cans Heather collects is the independent variable because it affects the amount of money she earns.

Step 3 ：The more cans she collects, the more money she earns.

Step 4 ：Therefore, the independent variable is the number of cans she collects.

Step 5 ：\(\boxed{\text{The independent variable represents the number of cans Heather collects.}}\)