One USB lead on the Android headunit has 4 pins, and the other has 6 pins; why?
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 device’s internal storage
The 6-pin connector on the USB lead only uses 5 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 connect to storage devices, mice, keyboards, and digital cameras.
The USB OTG is much more helpful to an Android headunit (like a phone) because it needs access to external USB storage, and even mice and keyboards can be helpful 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 to try. You can’t damage the Android headunit by plugging a device into the wrong USB socket, and it will either work or won’t.
Why can’t they all be USB OTG?
USB On-the-go was devised in 2001 to expand the capabilities of the early USB specification. There may be some devices that people will try to connect that don’t work well with the later addition to the USB standard. No problem because a basic 4-pin, fully pre-2001 compliant USB socket is available.
If you want to get into all the ins and outs, check out the Wikipedia article on USB OTG