\subsubsubsection doesn't exist but, instead, you can use \paragraph and \subparagraph, which will generate something like 1.1.1.1 and 1.1.1.1.1. .
To include this in your table of contents, you have to declare \setcounter{tocdepth}{4} and \setcounter{secnumdepth}{4}. For subparagraph, use {5}.
There are 7 levels of sections (depending on the document)
-1
\part{part}0
\chapter{chapter}1
\section{section}2
\subsection{subsection}3
\subsubsection{subsubsection}4
\paragraph{paragraph}5
\subparagraph{subparagraph}
\part and \chapter are only available in report and book.