I managed to flash the backup BIOS, (with a UD5; the Ud4 shouldn't be any different) using my MS sidewinder X4 usb keyboard, but I had to mash the alt f12 buttons fast and very quickly, like, before I even got the drives notice (maybe before the CPU string appeared). So no, you don't need a PS/2 keyboard to flash. But perhaps ps/2 is easier since it isn't initialized in the same way like the USB devices are...