\subsubsubsectionLaTeX's set of "sections" stops at the level of
\subsubsection. This reflects a design decision by Lamport -
for, after all, who can reasonably want a section with such huge
strings of numbers in front of it?
In fact, LaTeX standard classes do define "sectioning"
levels lower than \subsubsection, but they don't format them like
sections (they're not numbered, and the text is run-in after the
heading). These deeply inferior section commands are \paragraph
and \subparagraph; you can (if you must) arrange that these
two commands produce numbered headings, so that you can use them as
\subsubsubsections and lower.
The titlesec allows you to adjust the definitions of the
sectioning macros, and it may be used to
transform a \paragraph's typesetting so that it looks like that
of a \section.
If you want to program the change yourself, you'll find that the
commands (\section all the way down to \subparagraph) are
defined in terms of the internal \@startsection command, which
takes 6 arguments. Before attempting this sort of work, you are well
advised to read the LaTeX sources (ltsect.dtx in the
LaTeX distribution) and the source of the standard packages
(classes.dtx).
The
LaTeX companion discusses use of
\@startsection for this sort of thing.