Sneaking fourth chapter by Potter author (8)

I believe the answer is:

prowling

I'm a little stuck... Click here to teach me more about this clue!

'sneaking fourth chapter' is the definition.
'prowling' can be an answer for 'sneaking' (thesaurus). I'm unsure of the 'fourth chapter' bit.

'potter author' is the wordplay.
'potter' becomes 'p' (this might be a standard abbreviation of which I'm unaware).
'author' becomes 'Rowling' (JK Rowling).
'p'+'rowling'='PROWLING'

'by' is the link.

Can you help me to learn more?

(Another definition for prowling that I've seen is " moving stealthily".)

I've seen this clue in the Sydney Morning Herald.

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