main.dart

class _MainPageState extends State<MainPage> {
  List<Note> notes = initNotes();

  @override
  Widget build(BuildContext context) {
    return ...
          Row(
            children: <Widget>[
              _drawLine(0),
              LineDivider(),
              _drawLine(1),
              LineDivider(),
              _drawLine(2),
              LineDivider(),
              _drawLine(3),
            ],
  }

  _drawLine(int lineNumber) {
    return Expanded(
      child: Line(
        lineNumber: lineNumber,
        currentNotes: notes.sublist(0, 4),
      ),
    );
  }
}