when did patient 55700 visit the hospital via clinic referral/premature, the first time since 4 years ago?
select admissions.admittime from admissions where admissions.subject_id = 55700 and admissions.admission_location = 'clinic referral/premature' and datetime(admissions.admittime) >= datetime(current_time,'-4 year') order by admissions.admittime asc limit 1