what are the four most frequent drugs that were prescribed in the same hospital encounter to patients aged 60 or above after they had been diagnosed with ac posthemorrhag anemia, until 2104?
select t3.drug from ( select t2.drug, dense_rank() over ( order by count(*) desc ) as c1 from ( select admissions.subject_id, diagnoses_icd.charttime, admissions.hadm_id from diagnoses_icd join admissions on diagnoses_icd.hadm_id = admissions.hadm_id where diagnoses_icd.icd9_code = ( select d_icd_diagnoses.icd9_code from d_icd_diagnoses where d_icd_diagnoses.short_title = 'ac posthemorrhag anemia' ) and strftime('%y',diagnoses_icd.charttime) <= '2104' ) as t1 join ( select admissions.subject_id, prescriptions.drug, prescriptions.startdate, admissions.hadm_id from prescriptions join admissions on prescriptions.hadm_id = admissions.hadm_id where admissions.age >= 60 and strftime('%y',prescriptions.startdate) <= '2104' ) as t2 on t1.subject_id = t2.subject_id where t1.charttime < t2.startdate and t1.hadm_id = t2.hadm_id group by t2.drug ) as t3 where t3.c1 <= 4