%\documentclass[tikz,convert={outfile=\jobname.svg},border=5pt]{standalone}
\documentclass[tikz,border=5pt]{standalone}
\usepackage[utf8x]{inputenc}
% To create the .png:
% > pdflatex fig1.tex
% > convert -density 300 fig1.pdf -quality 100 fig1.png

\begin{document}
    \begin{tikzpicture}[font=\tiny]
        \tikzstyle{node_style} = [draw=white, very thick, circle, fill=orange]
        \tikzstyle{arrow_style1} = [->, black, line width=1, >=latex]
        \tikzstyle{arrow_style2} = [->, black, line width=2, >=latex]
        \tikzstyle{arrow_style3} = [->, black, line width=3, >=latex]


        \node[node_style] (n1) at (0,0) {A};
        \node[node_style] (n2) at (4,0) {B};
        \node[node_style] (n3) at (4,2) {C};
        \node[node_style] (n4) at (0,2) {D};

        \draw[arrow_style1] (n1) edge [bend left=20] (n2);
        \draw[arrow_style2] (n2) edge [bend left=20] (n1);
        \draw[arrow_style1] (n2) edge [bend left=20] (n3);
        \draw[arrow_style2] (n3) edge [bend left=20] (n2);
        \draw[arrow_style1] (n3) edge [bend left=20] (n4);
        \draw[arrow_style2] (n4) edge [bend left=20] (n3);
        \draw[arrow_style1] (n4) edge (n1);
        \draw[arrow_style3, shorten <=2pt, shorten >=2pt] (n2) edge [out=140, in=-60] (n4);
    \end{tikzpicture}

\end{document}
