When A pages are locked and you insert enough material between Page 4 and Page 4A to generate a new page, that new page will be listed with a lowercase letter (the new "a" page) followed by the uppercase letter (the existing "A" page).
Note that this same logic applies to scene numbering: If you've numbered your scenes and you add another scene between, for example, Scene 4 and Scene 4A, when you add scene numbers (keeping the existing numbers) the new scene will be 4aA.
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