SQL> with
  2    data as (
  3      select 'JV/34' v from dual
  4      union all
  5      select 'JV/034' v from dual
  6    ),
  7    split as (
  8      select v,
  9             substr(v,1,instr(v,'/')) first_part,
 10             to_number(substr(v,instr(v,'/')+1)) last_part
 11      from data
 12    )
 13  select v, first_part||to_char(last_part,'fm0000') res
 14  from split
 15  /
V      RES
------ ------------
JV/34  JV/0034
JV/034 JV/0034

with
      data as (
        select 'JV/34' v from dual
        union all
        select 'JV/034' v from dual
        union all
        select 'JV/0034' v from dual
      ),
      split as(
       select v,
              regexp_substr(v,'[^/]([A-Z]|[a-z])*') as first_part,
              regexp_substr(v,'[0-9].*') as last_part
       from data
       )
    select  v,first_part,last_part,first_part || '/' || lpad(last_part,4,0) as str
    from    split

with data as (
              select 'JV/34' v from dual union all
              select 'JV/034' v from dual union all
              select 'JV/0034' v from dual
             )
select  v,
        regexp_replace(regexp_replace(v,'/','/000'),'\d*(\d{4})','\1') new_v
  from  data
/

V       NEW_V
------- ----------
JV/34   JV/0034
JV/034  JV/0034
JV/0034 JV/0034

SQL>