what was the last time until 48 months ago patient 4718 was prescribed a medication via a tt route?
select prescriptions.startdate from prescriptions where prescriptions.hadm_id in ( select admissions.hadm_id from admissions where admissions.subject_id = 4718 ) and prescriptions.route = 'tt' and datetime(prescriptions.startdate) <= datetime(current_time,'-48 month') order by prescriptions.startdate desc limit 1