instruction: since 4 years ago, when was the last time that patient 027-178495 was diagnosed with acute copd exacerbation?
response: SELECT diagnosis.diagnosistime FROM diagnosis WHERE diagnosis.patientunitstayid IN (SELECT patient.patientunitstayid FROM patient WHERE patient.patienthealthsystemstayid IN (SELECT patient.patienthealthsystemstayid FROM patient WHERE patient.uniquepid = '027-178495')) AND diagnosis.diagnosisname = 'acute copd exacerbation' AND DATETIME(diagnosis.diagnosistime) >= DATETIME(CURRENT_TIME(), '-4 year') ORDER BY diagnosis.diagnosistime DESC LIMIT 1