Crossword Genius

Four Roman cardinals get together for a beer (4)

Ross

I believe the answer is:

mild

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'four' is the definition.
Both the definition and answer are adjectives. Perhaps they are linked in a way I don't understand?

I cannot understand how the remainder of the clue works.

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(Other definitions for mild that I've seen before include "Not severe or harsh" , "old cardinals ..." , "Not intense; beer" , "Gentle - bland" , "Moderately warm - type of beer" .)

I've seen this clue in The Independent.
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