\documentclass[oneside]{book}
\usepackage[utf8]{inputenc}
\usepackage{geometry}
\geometry{
  a4paper,
  total={210mm,297mm},
  left=15mm,
  right=15mm,
  top=15mm,
  bottom=15mm,
}
\setlength{\columnsep}{0pt}

\usepackage{courier}
\usepackage{multicol}

\begin{document}
\begin{multicols}{4}
\texttt{
\begin{scriptsize}
\include{index-joined}
\end{scriptsize}
}
\end{multicols}{4}
\end{document}
