请问delphi高手

请教delphi高手
有如下一段程序，运行时，在第一个窗口按按钮能打开第二个窗口，在第二个窗口按第一个按钮能打开第三个窗口，但在第二个窗口按第二个按钮不能打开第四个窗口，不知何解，望有高手能指出。
program Project1;

uses
  Forms,
  Unit1 in 'Unit1.pas' {Form1},
  Unit2 in 'Unit2.pas' {Form2},
  Unit3 in 'Unit3.pas' {Form3},
  Unit4 in 'Unit4.pas' {Form4};

{$R *.res}

begin
  Application.Initialize;
  Application.CreateForm(TForm1, Form1);
  Application.Run;
end.

//第一单元

unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;

type
  TForm1 = class(TForm)
  Button1: TButton;
  Label1: TLabel;
  procedure Button1Click(Sender: TObject);
  private
  { Private declarations }
  public
  { Public declarations }
  end;

var
  Form1: TForm1;

implementation
uses unit2;
{$R *.dfm}

procedure TForm1.Button1Click(Sender: TObject);
var
  form2 : TForm2;
begin
form2 := TForm2.Create(self);
form2.ShowModal;
form2.free;


end;

end.


//第二单元

unit Unit2;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, ExtCtrls,StdCtrls;

type
  TForm2 = class(TForm)
  Button1: TButton;
  Label2: TLabel;
  Button2: TButton;
  procedure Button1Click(Sender: TObject);
  procedure Button2Click(Sender: TObject);
  private
  { Private declarations }
  public
  { Public declarations }
  end;

var
  Form2: TForm2;

implementation
 uses unit3, unit4;
{$R *.dfm}

procedure TForm2.Button1Click(Sender: TObject);
var
  form3 : TForm3;
begin
form3 := TForm3.Create(self);
form3.ShowModal;
form3.free;
end;

procedure TForm2.Button2Click(Sender: TObject);
var
  form4 : TForm4;
begin
form4 := TForm4.Create(self);
form4.ShowModal;
form4.free;
end;

end.


//第三单元


unit Unit3;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;

type
  TForm3 = class(TForm)
  Label1: TLabel;
  private
  { Private declarations }
  public
  { Public declarations }
  end;

var
  Form3: TForm3;

implementation

{$R *.dfm}

end.


//第四单元

unit Unit4;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;

type
  TForm4 = class(TForm)
  Label1: TLabel;
  private
  { Private declarations }
  public
  { Public declarations }
  end;

var
  Form4: TForm4;

implementation

{$R *.dfm}

end.

[解决办法]
没有问题,不过这些定义都可以省去
var
form2 : TForm2;
form3 : TForm3;
form4 : TForm4;


[解决办法]
你的第二个按钮没有关联事件到Button2Click吧？