Signature in preface

\documentclass{article}

\def\sig#1{\vbox{\hsize=5.5cm
    \kern2cm\hrule\kern1ex
    \hbox to \hsize{\strut\hfil #1 \hfil}}}

\newcommand\signatures[4]{%
\hbox to \hsize{\hfil #1, \today\hfil}
\hbox to \hsize{\quad\sig{#2}\hfil\hfil\sig{#3}\quad}
\hbox to \hsize{\hfil\sig{#4}\hfil}}


\begin{document}
\signatures{St. Anford}{John Doe}{Jane Doe}{James Doe}
\end{document}

Each signature line is 5.5cm long and you might want to change this in the definition of \sig. Likewise, each signature line provides 2cm of vertical space for the actual signature. One \quad of space is left to the margins - you could use here anything like \hskip1cm as well.

One possible solution would be to use a tabular environment (change the lengths according to your needs):

\documentclass[10pt,b5paper,twoside]{report}
\usepackage[lmargin=25mm,rmargin=25mm,tmargin=27mm,bmargin=30mm]{geometry}
\usepackage{array}

\newcommand\Signature[4]{\par\bigskip
  \setlength\tabcolsep{0pt}
  \begin{tabular}{@{\hspace{.05\textwidth}}>{\centering\arraybackslash}p{.4\textwidth}
      @{\hspace*{.1\textwidth}}>{\centering\arraybackslash}p{.4\textwidth}@{\hspace{.05\textwidth}}}
  \multicolumn{2}{c}{#1, \today} \\[10ex]
  \rule{.4\textwidth}{0.4pt} & \rule{.4\textwidth}{0.4pt} \\
  #2 & #3 \\[10ex]
  \multicolumn{2}{c}{\rule{.4\textwidth}{0.4pt}} \\
  \multicolumn{2}{c}{#4}
\end{tabular}
\par
}
\begin{document}

\Signature{G\"ottingen}{David Hilbert}{Emmy Noether}{Hermann Minkowski}

\end{document}