SQL & PL/SQL :: ORDER BY Clause And ROWNUM

When i try to execute a query, which is organised as the below example, it retrieves data..

select * from (
select col1, col2, col3, col4....coln
from TABLE_ONE left outer join TABLE_TWO
-- some conditions and group by clause
order by 1 asc
)
where rownum <=1000;

Again if I use Column alias in the ORDER BY clause col1, the query won't retrieve data.

Also If I use ORDER BY 4 instead of ORDER BY 1, the query wont return data...

select * from (
select col1, col2, col3, col4....coln
from TABLE_ONE left outer join TABLE_TWO
-- some conditions and group by clause
order by 4 asc
)
where rownum <=1000;

The whole issue revolves around the inner ORDER BY Clause and external ROWNUM condition..If I eliminate any of the two, the query works fine...I am not sure if indexes have some role to play in it...