I have found my answer!!!
select emp_id, substr(sal_date,1,3) monthly, salary from (select emp_id, sal_date, salary, max(salary)over (partition by substr(sal_date,1,3)) max_sal from emp_salary order by emp_id) where salary=max_sal;
Result-set:
EMP_ID MONTHLY SALARY
Emp1 Mar 4000 Emp1 Feb 3000 Emp2 Jan 6000 Emp2 Apr 5000 Emp4 Dec 5000