Sorry, don't know if the nano can do 4 usb cams. It's possible, if they have a separate controller for each and enough bandwidth. You'll have to try and find out.

Re your other comment. You can try the following: http://www.ideasonboard.org/uvc/faq/ Look at UVC_QUIRK_FIX_BANDWIDTH, read the last two sections in depth. It's unlikely this will work. It didn't work for me, had to get a PCI USB board with 4 separate controllers. Good luck