how many patients have had a blood culture ( myco/f lytic bottle) test since 4 years ago?
select count( distinct admissions.subject_id ) from admissions where admissions.hadm_id in ( select microbiologyevents.hadm_id from microbiologyevents where microbiologyevents.spec_type_desc = 'blood culture ( myco/f lytic bottle)' and datetime(microbiologyevents.charttime) >= datetime(current_time,'-4 year') )