%%
%% The LaTeX Companion, 3ed
%%
%% Example 3-3-38 on page I-176 in "Customizing units and quantities".
%%
%% Copyright (C) 2022 Frank Mittelbach
%%
%% It may be distributed and/or modified under the conditions
%% of the LaTeX Project Public License, either version 1.3c
%% of this license or (at your option) any later version.
%%
%% See https://www.latex-project.org/lppl.txt for details.
%%

\documentclass{tlc3exa}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{87.0pt}

%StartShownPreambleCommands
\usepackage{siunitx}   \DeclareSIPower\quartic\tothefourth{4}
%StopShownPreambleCommands

\begin{document}
\unit{\newton\tothefourth} and \unit{\quartic\metre}
\end{document}