\begin{tikzpicture}[node distance=2cm]
  \node (3) [draw,circle, fill=yellow] {};
  \node (0) at (0.2,-2) [draw,circle,fill=blue]{}; 
  \node (4) at (0,-1.8) [draw,circle,fill=red,semitransparent]{};
  \node (2) [draw,circle,fill=magenta, right of=3]{};
  \node (1) [draw,circle,fill=green, below of=2]{};
  \path[->] (0) edge (1);
  \path[->] (1) edge (2);
  \path[->] (2) edge (3);
  \path[->] (3) edge (4);
  \node (flux) at (1,-1) [] {$\Phi$};
 \end{tikzpicture}
