what is the first care unit for patient 4718 until 4 years ago?
select transfers.careunit from transfers where transfers.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 4718 ) and transfers.careunit is not null and datetime(transfers.intime) <= datetime(current_time,'-4 year') order by transfers.intime asc limit 1