If you use References | Table of Contents | Insert Table of Contents...,
you'll get the familiar dialog that (among other things) allows you to
select the number of levels. If you've used one of the TOCs from the
gallery, press Alt+F9 to view the field code and edit the \o "1-3" switch to
\o "1-4".

In addition to what Suzanne suggested, don't forget to actually add level 4
text to the document, for example by applying the Heading 4 built-in style
to at least one paragraph.