매크로로 감싼버전

\documentclass[a4paper,12pt]{article}
\usepackage{amsmath,amsthm,kotex}

\newtheorem{thm}{Theroem}
\let\origthethm=\thethm
\def\MakeThmMultiple#1{%
    \def\thethm{\arabic{thm}\kern2pt--\kern2pt\addtocounter{thm}{\the\numexpr #1 - 1\relax}\arabic{thm}}%
}
\def\MakeThmSingle{%
  \let\thethm=\origthethm%
}

\title{Multiple indices for a theorem}

\begin{document}
\maketitle
\begin{thm}[A single theorem]
    I am a theorem.
\end{thm}

\MakeThmMultiple{4}

\begin{thm}[Multiple theorems]
    I am a collection of theorems.
\end{thm}

\MakeThmSingle


\begin{thm}[Another single theorem]
    I am another theorem.
\end{thm}
\end{document}