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First of all, change PasswordChar property of Edit component that nobody saw your password, when you type it.
In this example, length of password must be only 4 symbols.

procedure TForm1.Edit1Change(Sender: TObject);
begin
  with Sender as TEdit do
    if Length(Text)>4 then
    begin
      Edit1.Text:='';
      MessageBeep(0);
    end;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  MessageBox(
    Form1.Handle, 
    PChar('You have just written - '+Edit1.Text),
    'Password checking',
    1);
end;

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