\documentclass[a4paper,12pt]{report} 
\usepackage{tikz,amsmath,amssymb}
\tikzstyle{vertex}=[circle, draw, inner sep=0pt, minimum size=12pt] 
\newcommand{\vertex}{\node[vertex]}
\tikzstyle{vertexs}=[draw,minimum width=.4cm,minimum height=.4cm]
\newcommand{\vertexs}{\node[vertexs]}
\begin{document}
\begin{figure}[htbp]
\[
\begin{tikzpicture}[scale=1,thick,>=stealth] 
\vertex (a1) at (0,0) []{};
\vertex (a2) at (0,-1) []{};
\vertex (a3) at (0,-2) []{};
\vertex (a4) at (0,-3) []{};
\vertexs (b1) at (2,-1) []{};
\vertexs (b2) at (2,-2.1) []{};
\draw [>"1"] (a1) -- (b1);
\draw [>"2"] (a1) -- (b2);
\draw [>"3"] (a2) -- (b2);
\draw [>"3"] (a3) -- (b1);
\draw [>"2"] (a4) -- (b1);
\draw [>"1"] (a4) -- (b2);
\end{tikzpicture}\]
\caption{}
\end{figure}
\end{document}