since 4 years ago, what was the four most frequently prescribed drugs for the patients of the 50s in the same month after they had been diagnosed with chr blood loss anemia?
select t3.drug from ( select t2.drug, dense_rank() over ( order by count(*) desc ) as c1 from ( select admissions.subject_id, diagnoses_icd.charttime 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 = 'chr blood loss anemia' ) and datetime(diagnoses_icd.charttime) >= datetime(current_time,'-4 year') ) as t1 join ( select admissions.subject_id, prescriptions.drug, prescriptions.startdate from prescriptions join admissions on prescriptions.hadm_id = admissions.hadm_id where admissions.age between 50 and 59 and datetime(prescriptions.startdate) >= datetime(current_time,'-4 year') ) as t2 on t1.subject_id = t2.subject_id where t1.charttime < t2.startdate and datetime(t1.charttime,'start of month') = datetime(t2.startdate,'start of month') group by t2.drug ) as t3 where t3.c1 <= 4