\documentclass[a4paper]{article}

\usepackage[final]{pdfpages}

\newcommand*{\numberofpages}[1]{%
  \the\XeTeXpdfpagecount"#1" %
}

\newcounter{numberofsignaturepages}%

\begin{document}
  \setcounter{numberofsignaturepages}{\numberofpages{yokutango.pdf}}%
  \addtocounter{numberofsignaturepages}{4}%
  \includepdf[pages=-,nup=1x2,landscape,signature=\thenumberofsignaturepages]{yokutango.pdf}
\end{document}
