The answer to the question, if there is a font size sammler thean tiny, depends on the used font. By using the default font you won't get a font size smaller than tiny. One have to include the usepackage lmodern for example.
\documentclass{article}
…
\usepackage{lmodern}
…
\begin{document}
…
\end{document}
After this is done, the following command can be used:
{\fontsize{4}{12} \selectfont smaller than tiny}
Note only the text within curly braces is smaller, the rest of the text will have normal size.