When org-mode exports content to HTML it converts headlines to
<hN> tags. The default headline level is 3; any headline after that will be wrapped in a
<li> tag instead.
org-export-headline-levels will change this behaviour.
Adding the following to your init.el will wrap
level 4 in a
(setq org-export-headline-levels 4)
Alternatively this can be done on a per-file basis using
#+OPTIONS somewhere in the org file. The same headline export settings as above would look like this:
#+OPTIONS: H:4 * level 1 ** level 2 *** level 3 **** level 4
The org-mode manual has more information on org-mode export settings.