Four astute tradesmen's stocks (6)

I believe the answer is:

tetrad

'four' is the definition.
(synonyms)

'astute tradesmen's stocks' is the wordplay.
'stocks' says the answer is hidden in the clue.
'TETRAD' is hidden within 'astute tradesmens'.

(Other definitions for tetrad that I've seen before include "quartet" , "Group or set of four" , "number of Gospel writers" .)

I've seen this clue in The Independent.

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