Apparently it was far less common to have a quad 20mm than a single 37mm, but it did happen:
https://uboat.net/technical/flak.htm
Quote:
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Some boats had the 3.7cm gun removed and featured the quadruple 2cm 38/43 U with shield.
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I can’t find any pictures of non-u-flak u-boats with the flakvierling other than the one of U-745 at the link above.
If you’re debating with yourself about wether to use the flakvierling, I say go for it. Unless all the AI u-boats have one as well, you’re still being perfectly historically authentic, at least if you’re using a type VII boat.