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.

Altering org-export-headline-levels will change this behaviour.

Adding the following to your init.el will wrap level 4 in a <h5> tag:

(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:


* level 1
** level 2
*** level 3
**** level 4

The org-mode manual has more information on org-mode export settings.