Osmosis is the process of water molecules moving from an area of higher concentration to an area of lower concentration. This movement occurs through a semi-permeable membrane. A semi-permeable membrane allows certain particles, in this case water molecules, to pass through while blocking the passage of others.

Let`s analyze each option to better understand why option 4 is the correct answer:

- Option 1 states that osmosis occurs through a cell wall. However, osmosis actually happens through a semi-permeable membrane, not a cell wall. Therefore, option 1 is incorrect.

- Option 2 suggests that water molecules move from an area of lower concentration to an area of higher concentration. This contradicts the definition of osmosis, where water molecules flow from higher concentration to lower concentration. Thus, option 2 is incorrect.

- Option 3 states that osmosis occurs through a fully permeable membrane. However, osmosis specifically relies on a semi-permeable membrane to allow the movement of water molecules. Therefore, option 3 is incorrect.

- Option 4 accurately describes osmosis, as it states that water molecules move from an area of higher concentration to an area of lower concentration through a semi-permeable membrane.