In addition to \part, \section, \subsection and \subsubsection, in the article document class, there exist the predefined commands \paragraph and \subparagraph. Give them a try before defining new sectioning commands.
The code in the first post does not work since all the subsubsubsection stuff should be in between the \makeatletter and \makeatother commands. By the way, the \bf command is obsolete and \subsubsubsectionmark is not needed, since no pagestyle contains that command (not at least the standard ones).
Before introducing a new sectioning command in an article, in my opinion, one should consider other alternatives:
1) Move to the book or report classes. The \chapter command provides a new level.
2) Use \part.
3) Try \paragraph.
4) Reorganize the document.
If finally one really needs a new numbered level behaving like \section and like, the simplest approach is to redefine \paragraph and increment the counters secnumdepth and tocdeph, as the following example shows:
Juanjo, thanks for your suggestion--it works very well. I have a related issue that is driving me crazy.
I structure my report using \section, \subsection, \subsubsection and \paragraph commands. I like the paragraphs, but the paragraph numbering usually continues across sections and subsections. What is the method to "reset" this paragrapah numbering such that it starts fresh in each section or subsection?
to reset the paragraph counter every time a new section is produced (replace "section" with "subsection" in the code above to reset the paragraph counter every time a new subsection is produced).
subordinates the paragraph counter to the subsubsection one, but it also redefines \theparagraph. If you want to preserve your redefinition of \theparagraph, simply use the starred version of \counterwithin, that is, \counterwithin*.