[O] How to make the 4 level heading '****' to 'subsubsection' in latex export

[XP Chen]

I add follow lines in .emacs:

(add-to-list 'org-latex-classes
             '("thesis1"
               "\\documentclass{thesis1}"
("\\chapter{%s}" . "\\chapter*{%s}")
("\\section{%s}" . "\\section*{%s}")
("\\subsection{%s}" . "\\subsection*{%s}")
("\\subsubsection{%s}" . "\\subsubsection*{%s}")
("\\paragraph{%s}" . "\\paragraph*{%s}")))
("\\subparagraph{%s}" . "\\subparagraph*{%s}")))

However it no works. The .tex have no subsubsection. Instead, it convert "**** XX" to "\item XX" in .tex file.

Is the org export can only accept 3 section? How to resolve it?

Thanks.

                                                                 Chen.