\begin{figure}
  \centering
  % \begin{tikzpicture}
  %   \draw[step=1,help lines] (-4,-4) grid ( 4, 4);
  %   \fill[red] (0, 0) circle (5pt);

  %   \draw[dashed, rotate=60] (-3, 0) ellipse (1.5 and 0.5);
  %   \draw[dashed, rotate=300] (3, 0) ellipse (1.5 and 0.5);

  %   \fill[blue!50, ] (1,1) circle (2pt);
  %   \fill[blue!50, ] (1.5,0.7) circle (3pt);
  %   \fill[blue!50, ] (1.2,0.9) circle (3pt);


  % \end{tikzpicture}
  \begin{tikzpicture}[auto, every loop/.style={},
      thick,
      main node/.style={circle,draw=none, fill=green!50},
      path/.style={circle, inner sep=2pt,fill=red!50},
      edge_style/.style={bend right, draw=orange, line width=2, ultra thick}]

    \node[main node, fill=red!50]                                (1) {};
    \node[main node, below= 1mm of 1]               (2) {};
    \node[main node, below left= -1mm and 3mm of 1] (3) {};
    \node[main node, below right=-1mm and 3mm of 1] (4) {};

    \draw[] (1) -- (2);
    \draw[] (1) -- (3);
    \draw[] (1) -- (4);
    \node[ellipse, x radius=2cm, y radius=0.5cm, draw=red,  rotate=30, dashed, fit=(1) (2) (3) (4), inner sep=1mm] (all) {};

    \node[main node, red, fill=gray, above right=5mm and 2cm of all] (3) {3};
    \node[main node, blue, fill=gray, below right=5mm and 2cm of all] (5) {5};

    \draw[bend right=30, looseness=1.25] (all) -- node [above] {4} (3);
    \draw[dashed] (all) -- node[below] {2} (5) -- node[right] {6} (3);
  \end{tikzpicture}

  % % https://q.uiver.app/? 
  % \[\begin{tikzcd}
  %     {\mathbf{C}} && {{\scriptsize Q}} & {\mathbf{D}} \\
  %     & {\mathbf{\hat{C}}} & {\mathbf{\hat{D}}} &&& {} \\
  %     && {{\scriptsize(Q_1, Q2)}}
  %     \arrow["{\mathsf{GW}}"{description}, curve={height=-18pt}, tail reversed, from=1-1, to=1-4]
  %     \arrow[curve={height=-12pt}, from=1-4, to=2-3]
  %     \arrow["{\mathsf{OGW_{lb}}}"', curve={height=12pt}, tail reversed, from=2-2, to=2-3]
  %     \arrow["{\mathsf{OGW_{ub}}}"', curve={height=12pt}, tail reversed, from=2-3, to=2-2]
  %     \arrow["{\mathbf{V}^\top \mathbf{C} \mathbf{V}}"{description}, curve={height=12pt}, from=1-1, to=2-2]
  %   \end{tikzcd}\]
  \caption{Upper and lower bounds of OGW}
  \label{fig:ogw_lb_ub}
\end{figure}