One USB lead on the android headunit has 4 pins, the other has 6 pins, why?
4 pins
The 4 pin connector is a standard USB connector and will charge your phone (slowly, probably not the best use for it) and connect to a host to expose the devices internal storage
6 pins
The 6 pin connector on the USB lead only uses 5 of those pins and does everything the 4 pin connector does. Plus:
The extra connection in the 6 way plug allows for USB OTG (USB on-the-go) which allows the headunit to also connect to storage devices, mice, keyboards and digital cameras.
The USB OTG is much more useful to an android headunit (which is like a phone) because it needs access to external USB storage and even mice and keyboards can be useful to navigate menus if the touch screen is unresponsive.
Tips for 4/6 pin USB
The tip is if you are unsure what will work in which USB socket is to just try. You can’t damage the android headunit by plugging a device into the wrong USB socket and it will either work, or it won’t.
Why can’t they all be USB OTG?
USB On-the-go was devised in 2001 to expand the capabilities on the early USB specification. There may be some devices out there that people will try to connect that just don’t work well with the later addition to the USB standard. No problem, because there is a basic 4 pin, fully pre-2001 compliant USB socket available.
Further reading
If you want to get into all the ins and outs, check out the wikipeda article on USB OTG
Thank! this was really helpful!