instruction: until 2102, what was the four of the most frequent output events?
response: SELECT t1.celllabel FROM (SELECT intakeoutput.celllabel, DENSE_RANK() OVER (ORDER BY COUNT(*) DESC) AS c1 FROM intakeoutput WHERE intakeoutput.cellpath LIKE '%output%' AND STRFTIME('%y', intakeoutput.intakeoutputtime) <= '2102' GROUP BY intakeoutput.celllabel) AS t1 WHERE t1.c1 <= 4