digraph G {
  splines = false;
  node [ shape = "circle" ];
  {
    rank = same;
    b1 [ label = "a" ];
    b2 [ label = "b" ];
  }
  {
    rank = same;
    b3 [ label = "c" ];
    b4 [ label = "d" ];
  }

  b1 -> b2;
  b2 -> b4;
  b1 -> b3;
  b3 -> b4;
  b4 -> b1;
}