\documentclass{book}

\usepackage{color}
\usepackage{graphics}
\usepackage{transparent}


\begin{document}



\cite{x1,x2,x3,x4} 

\begin{thebibliography}{99}

\bibitem{x1}
Farschad Troba

\bibitem{x2}
Farschad Troba

\bibitem{x3}
Farschad Troba

\bibitem{x4}
Farschad Torabi.

\end{thebibliography}


\end{document}