USB 3 is NOT 4 pin. USB2 uses 4 pins, and you can plug a USB3 connector into USB2, but if you look deep into a USB3 socket you'll see that there are extra pins that connect when you plug USB3 into USB3.
If your motherboard doesn't have a USB3 connector, then leave it unconnected.
Thank you for the reply! I have a Gigabyte GA-870A-USB3 mobo which you're right [NOW I looked at it] more pins for USB 3.0 than 2.0. The mobo also does not have a USB header interface [20 pin] connector on it. It does have just the USB 3.0 outside on the I/O panel though.
Just that this is a new project, and I hate to buy a case which requires the USB 3.0 cable to come out from the back, and plug into the external interface.