instruction: since 2 years ago, what were the top four most frequently prescribed drugs for patients with age 40s?
response: SELECT t1.drugname FROM (SELECT medication.drugname, DENSE_RANK() OVER (ORDER BY COUNT(*) DESC) AS c1 FROM medication WHERE medication.patientunitstayid IN (SELECT patient.patientunitstayid FROM patient WHERE patient.age BETWEEN 40 AND 49) AND DATETIME(medication.drugstarttime) >= DATETIME(CURRENT_TIME(), '-2 year') GROUP BY medication.drugname) AS t1 WHERE t1.c1 <= 4