DECLARE
  G_add varchar2(100);
  G_ORDER_DATE_FROM Date;
  G_ORDER_DATE_TO Date;
  G_INS_DATE_FROM Date;
  G_INS_DATE_TO Date;
  
BEGIN
  :G_ORDER_DATE_FROM := '&&1';
  :G_ORDER_DATE_TO := '&&2';
  :G_INS_DATE_FROM := '&&3';
  :G_INS_DATE_TO := '&&4';

if :G_INS_DATE_FROM is null then

G_add := 'and (to_date(install_date) between  nvl(:G_INS_DATE_FROM,install_date) and nvl(:G_INS_DATE_TO,install_date))';

else

G_add := ' ';

end if;				    

select order_number, line_number, order_line_id
     , part_number, item_description,install_date
     , sum(decode(category_type,'ILMI',1,'ILM',1,0)) ilm_count
     , sum(decode(category_type, 'ITM',1,0)) itm_count
     , sum(decode(category_type,'IDT',1,0)) idt_count
  from atrd_serial_num_ship_v sn
 where date_ordered between nvl(:G_ORDER_DATE_FROM,date_ordered)
                        and nvl(:G_ORDER_DATE_TO,date_ordered)
   and decode(:G_INS_DATE_FROM,null,null,.......

if :G_INS_DATE_FROM is not null then we need to get the G_add statement in this place. I tried as  ||&G_add  but it never worked.

group by order_number, line_number, order_line_id
      , part_number, item_description,install_date;
END;