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;