can not execute LIMIT on sql Developer
Oracle 11g doesn't support the LIMIT clause, though the impending 12c release is rumored to support it.

Anyway, you can do this using an analytic windowing function:

select * from
( 
  select salary, row_number() over (order by salary desc) as rn
  from emp
)
where rn = 4;


You can also do this using rownum, but I find the above way to look cleaner.

Example SQL Fiddle for you to mess around with.