Try just using -usbport, for each command run:
-usbport 1
-usbport 2
-usbport 3
-usbport 4
Works for me, even though windows reports the usb COM port to be 43, 47, 51, 62, etc. I’ve done this with the EM7565 and flashed 4 at once on Windows 10 with a powered usb hub.
Make sure there is a space in there in: -usbport 1
Here is an example of one of my commands:
fdt2.exe -d g5k -f fw1.cwe fw2.cwe pri1.nvu pri2.nvu -usbport 1 -multi -mi -impref fw1.cwe pri1.nvu