That's a great question! Here's the breakdown of the levels of organization in living things:

1. Cell: The basic, fundamental unit of life.

2. Tissue: A group of similar cells working together to perform a specific function (e.g., muscle tissue, nervous tissue).

3. Organ: A structure made up of different tissues that work together to perform a more complex function (e.g., heart, lungs, stomach).

4. Organ System: A group of organs that work together to carry out major bodily functions (e.g., digestive system, circulatory system, nervous system).

So, the fourth level of organization in living things is the organ system.