Используйте скобки:
SELECT event_datetime as day, event_param_2 as R1, event_param_3 as Content, event_code
FROM events
WHERE upcNameId = 'ExampleName'
AND event_param_2 = 1
AND event_datetime >= '2017-01-01 00:00:00'
AND event_datetime <= '2018-04-01 00:00:00'
AND (event_code = 51 OR event_code = 52 OR event_code = 53 OR event_code = 54)
В качестве альтернативы используйте IN
:
SELECT event_datetime as day, event_param_2 as R1, event_param_3 as Content, event_code
FROM events
WHERE upcNameId = 'ExampleName'
AND event_param_2 = 1
AND event_datetime >= '2017-01-01 00:00:00'
AND event_datetime <= '2018-04-01 00:00:00'
AND event_code IN(51, 52, 53, 54)