Aux V on the M4 is pin 18. I would only recommend this if your wide band meter has an auxilluary voltage output (which I assumed
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it didn't but a quick check of the website suggests that some of them do), but having had some experience calibrating sensors on my M48 I suspect it's a: not going to be easy and b: not particularly worthwhile (could be wrong though
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). You also won't be able to take advantage of the lambda-specific logging options and will only get a numerical representation that you will have to convert to afr.
IIRC the wide lambda option is about $900 NZ.
Also newer M4s don't need the Motec ECU cable, you can use a standard serial cable.