tell me the top four drugs that are most frequently prescribed for patients in the 60 or above?
select t1.drug from ( select prescriptions.drug, dense_rank() over ( order by count(*) desc ) as c1 from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.age >= 60 ) group by prescriptions.drug ) as t1 where t1.c1 <= 4