\documentclass{article}
\usepackage[pdftex,active,tightpage]{preview}
\setlength\PreviewBorder{2mm}
\usepackage{tikz}
\usetikzlibrary{arrows,positioning, calc}
\tikzset{
    %Define standard arrow tip
    ->,>=stealth',shorten >=1pt,auto,node distance=3cm, very thick,
    % Define arrow style
    main node/.style={
        circle,
        fill=blue!20,
        draw,
        font=\sffamily\Large\bfseries
    }
}

\begin{document}
\begin{preview}
\begin{tikzpicture}[,]

  \node[main node] (A) {A};
  \node[main node] (C) [below of=A] {C};
  \node[main node] (B) [right of=A] {B};
  \node[main node] (D) [right of=C] {D};

  \path[every node/.style={font=\sffamily\small}]
    (A) edge node {4} (C)
        edge node {3} (B)
    (C) edge node {-2} (B)
    (B) edge node {1} (D);
\end{tikzpicture}
\end{preview}
\end{document}