instruction: give me the top four most frequently prescribed drugs for patients with 50s in this year.
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 50 AND 59) AND DATETIME(medication.drugstarttime, 'start of year') = DATETIME(CURRENT_TIME(), 'start of year', '-0 year') GROUP BY medication.drugname) AS t1 WHERE t1.c1 <= 4