graph g {
    splines=false;
    node [ shape="circle" ];

    {
        rank="same";
        A; B;
    }
    {
        rank="same";
        C; D;
    }

    A -- B [ label="1" ];
    A -- C [ label="2" ];
    A -- D [ label="3" ];
    B -- D [ label="2" ];
}