Hunch. S&P added four companies at a single review in March 2025 and again in March 2026, so four is rare but has happened.
The four-or-more market resolves on the same December review release: YES if it names four or more companies joining the S&P 500, and NO if it names three or fewer or does not come out by 18 December. The rules on separate and mixed releases and the backups are the same as for the any-company market.
We looked for cases where S&P ran its regular quarterly review of the S&P 500 after at least two companies had already joined early in the same quarter. We started in September 2019, the first review S&P announced two weeks ahead. We found nine such reviews from 2019 to 2026, and counted how many companies each review added.