When creating the TOC, use the Table of Contents Options dialog box to add
styles. If you are using Word 2007, you will have to create the TOC via
References tab | Table of Contents | Insert Table of Contents; to open the
options dialog box, click the Options button.

If you are using Word 97-2003, see
http://sbarnhill.mvps.org/WordFAQs/TOCTips.htm, which also discusses the
dialog box (in any version).

To edit an existing TOC field: Press Alt+F9 to show field codes. The TOC
field will look similar to { TOC \o "1-3" \h }. Change "1-3" to "1-4" and
then press F9 to update. Use Alt+F9 to hide field codes again.