select tre.*, level as lvl, (CONNECT_BY_ROOT tre.id) as id_root
    , CONNECT_BY_ISLEAF as isleaf, CONNECT_BY_ISCYCLE iscycle
    , SYS_CONNECT_BY_PATH( tre.id, '/') as val
    from 
    (    select 1 as id, NULL as prev  from dual 
            union
        select 2 as id, 1 as prev from dual 
            union
        select 3 as id, 2 as prev from dual -- 1-2-3
            union
        select 4 as id, 1 as prev from dual -- 1-4
            union
        select 5 as id, 4 as prev from dual -- 1-4-5
    ) tre
where 2=2 --and level = 3
    START WITH prev IS NULL
    CONNECT BY nocycle PRIOR  id = prev
    order SIBLINGS by id

ID PREV  LVL ROOT LEAF CYCLE VAL
1  null   1    1    0    0    /1
2    1    2    1    0    0    /1/2
3    2    3    1    1    0    /1/2/3
4    1    2    1    0    0    /1/4
5    4    3    1    1    0    /1/4/5

select tre.*, level as lvl, (CONNECT_BY_ROOT tre.id) as id_root
    , CONNECT_BY_ISLEAF as isleaf, CONNECT_BY_ISCYCLE iscycle
    , SYS_CONNECT_BY_PATH( val, '/') as val2
    from 
    (    select 1 as id, NULL as prev, 1 as val  from dual UNION
        select 2 as id, 1 as prev, 2 as val  from dual  UNION -- 1-2 
        select 3 as id, 2 as prev, 3 as val  from dual  UNION -- 1-2-3
        select 8 as id, 3 as prev, 6 as val  from dual  UNION -- 1-2-3-7
        select 9 as id, 3 as prev, 7 as val  from dual  UNION -- 1-2-3-6
        select 4 as id, 1 as prev, 4 as val  from dual  UNION -- 1-4
        select 5 as id, 4 as prev, 5 as val  from dual  UNION -- 1-4-5
        select 7 as id, 5 as prev, 7 as val  from dual  UNION -- 1-4-5-7
        select 6 as id, 5 as prev, 6 as val  from dual  -- 1-4-5-6
    ) tre
where 2=2 --and level = 4  --and prev IS NOT NULL 
SYS_CONNECT_BY_PATH function is not allowed here
    START WITH prev IS NULL -- начать с листьев 
    CONNECT BY nocycle PRIOR  id = prev 
/**/    
        and (2=3
        -- Единый блок условий: взять левелы=4 с ... на 2-м левеле , у которых на 4-м левеле ...
        OR level=2 and val in (2,4)
        OR level between 3 and 3  -- сохраняет непрерывность уровней
        OR level=4 and val= 7 
        OR level>4              -- дальнейшие уровни
        )        
/**/    order SIBLINGS by id