just count the number of patients who had been prescribed syringe (neonatal) *d5w* until 4 years ago.
select count( distinct admissions.subject_id ) from admissions where admissions.hadm_id in ( select prescriptions.hadm_id from prescriptions where prescriptions.drug = 'syringe (neonatal) *d5w*' and datetime(prescriptions.startdate) <= datetime(current_time,'-4 year') )