\begin{tikzpicture}
  \tikzset{every node/.style={shape=circle,draw,fill=white,inner sep=2pt}}

\node (3) at (0, 0) {$3$};
\node (4) at (1, 0) {$4$};
\node (6) at (0, 1) {$6$};
\node (12) at (0, 2) {$12$};
\node (20) at (1, 1) {$20$};

\draw (3) -- (6);
\draw (4) -- (12);
\draw (4) -- (20);
\draw (6) -- (12);

\end{tikzpicture}