It's true that following Shauna Kelly's procedure for setting up multilevel numbering will fix the problem, and for long-term stability of the document that's recommended. For a quick fix, however, this will do:

• Right-click the Heading 4 box in the Styles gallery in the Home tab, and click Modify.
• In the Modify Style dialog, click the Format button in the bottom left corner and click Paragraph in the menu.
• In the Paragraph dialog, the second dropdown is labeled "Outline level". Change that dropdown from "Level 3" to "Level 4".
• Click OK in both dialogs.
• Right-click the Table of Contents and choose Update Field. In the Update box, choose to update the entire table.