Hi Richard, and welcome to the Forums. Have a look at the instructions I wrote in this reply, and give those steps a try: Jetson Nano 2gb not showing up on Dashboard - #5 by dtischler
That should get your Nano’s into a state where they can boot balenaOS.
Just be sure to look carefully at what device you are set to flash, since you mentioned yours are the 4gb variant, and the list of boards in the Nvidia SDK Manager all look fairly similar. 