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Very common for LiPo battery packs to have +, Clock, Data (for I2C) and -. Also on Canon battery packs and allows camera to see serial number.
Some Kindle internal packs are similar.
Larger packs also may have a connection to an onboard thermistor, especially laptop packs and some power tools.
I did wonder myself if it was USB, but thought it unlikely. Of course the similar 4 spots on some tablets are for USB, a keyboard dock.
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