Question: What about events that have 2 or 3 notes as well? Can you add those to the list.
SQL: SELECT T1.Customer_Event_ID  FROM Customer_Events AS T1 JOIN Customer_Event_Notes AS T2 ON T1.Customer_Event_ID   =   T2.Customer_Event_ID GROUP BY T1.customer_event_id HAVING count ( * )  BETWEEN 1 AND 3