This is causing too many problems in 4.0a as users are confused and think that they must Merge. Then they encounter index failures which can only be fixed by a full reindex.
Both the message and the section name are inappropriate. Please remove the former and rename the latter. Moreover, please hide that section until it is needed ie. search by Ranked.
Also, how stable is the Merge process in 4.0b? Can we attempt a merge with confidence?