new - application
add form 2,3 and 4
add combobox1 to form1, add 3 items form 2 form 3 form 4

add code to onselect of combobox1 on form1

procedure TForm1.ComboBox1Select(Sender: TObject);
begin
if combobox1.ItemIndex = 0 then
begin
  Form2.show;
  form3.hide;
  form4.hide;
end;
if combobox1.ItemIndex = 1 then
begin
  Form3.show;
  form4.hide;
  form2.hide;
end;
if combobox1.ItemIndex = 2 then
begin
  Form4.show;
  form3.hide;
  form2.hide;
end;
end;