How do I make a circle of fermions in feynmp? (a self-energy photon)

Question

I'm trying to draw a photon turning into a circle formed by one fermions going one way and the other going the other way, but they both go up. Here is my MWE:

\documentclass[amsmath,amssymb,aps,12pt,eqsecnum]{revtex4}  
\usepackage[a4paper]{geometry}  
\linespread{1}  
\usepackage[breaklinks, colorlinks, citecolor=blue]{hyperref}  
\usepackage{slashed}  
\usepackage{simplewick}  
\usepackage{feynmp-auto}  
\usepackage[pdftex]{graphicx}  
\usepackage{textcomp}  
\usepackage{gensymb}  

\begin{fmffile}{rad4}  
    \begin{fmfgraph*}(80,120)  
        \fmftop{t1,t2}  
        \fmfbottom{b1,b2}  
        \fmf{fermion}{b1,v1,t1}  
        \fmf{fermion}{b2,v4,t2}  
        \fmffreeze  
        \fmf{photon}{v1,v2}  
        \fmf{phantom}{v2,v3}  
        \fmf{photon}{v3,v4}  
        \fmffreeze  
        \fmf{fermion,left,tension=0}{v2,v3}  
        \fmf{fermion,right,tension=0}{v3,v2}  
    \end{fmfgraph*}  
\end{fmffile}  

This is what I'm currently getting, any ideas?
(i.e. I would like one of the fermions in the middle to go down and form a circle)
Thanks for the edit egreg!

Answer 1

Change that last fermion to

\fmf{fermion,left,tension=0}{v3,v2} 

Since it's going backwards from v3 to v2 it needs to bend left not right. Then you get:

Here's my complete, minimal compilable version of your picture:

\documentclass[border=1cm]{standalone}
\usepackage{feynmp-auto}  
\begin{document}
\begin{fmffile}{rad4}  
    \begin{fmfgraph*}(80,120)  
        \fmftop{t1,t2}  
        \fmfbottom{b1,b2}  
        \fmf{fermion}{b1,v1,t1}  
        \fmf{fermion}{b2,v4,t2}  
        \fmffreeze  
        \fmf{photon}{v1,v2}  
        \fmf{phantom}{v2,v3}  
        \fmf{photon}{v3,v4}  
        \fmffreeze  
        \fmf{fermion,left,tension=0}{v2,v3}  
        \fmf{fermion,left,tension=0}{v3,v2}  
    \end{fmfgraph*}  
\end{fmffile}  
\end{document}