Invalid identifier error on field created with select statement

Why sql bellow don't work?

select
    a.field1, a.field2, a.field3,
    (select count(*)
        from table2 b
        where b.field1 = a.field1
    ) as field4,
    (select count(*)
        from ta开发者_开发技巧ble3 b
        where b.field1 = a.field1
    ) as field5,
    (select count(*)
        from table4 b
        where b.field1 = a.field1
    ) as field6,
from table1 a
order by field4

Oracle says: ORA-00904: "field4": invalid identifier

---

try to wrap it up

select * from 
(    
select
    a.field1, a.field2, a.field3,
    (select count(*)
        from table2 b
        where b.field1 = a.field1
    ) as field4,
    (select count(*)
        from table3 b
        where b.field1 = a.field1
    ) as field5,
    (select count(*)
        from table4 b
        where b.field1 = a.field1
    ) as field6,
from table1 a
)
order by field4