since 2102, what were the four most frequently prescribed drugs for patients 20s?
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 between 20 and 29 ) and strftime('%y',prescriptions.startdate) >= '2102' group by prescriptions.drug ) as t1 where t1.c1 <= 4