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When can I claim a ghost or candy bonus?
The designer explained the "exactly 4, 6 or 9" rule in several replies from 2020 to 2021.
Only when you hold exactly 4, 6 or 9 ghosts (or candies), as the result of circling one. You may then claim that level's bonus or a lower one that is still available; at 5, 7 or 8 you can claim nothing. Several players can claim the same bonus, even in the same turn, but each player can take only one ghost bonus and one candy bonus per game. If you pass on a bonus, you can claim it later only while you still hold exactly that count; one more ghost removes the option until you reach the next level.