After making changes to an existing program I had written, which used CMSIS routines, I made a programming error after which the T4 swamped the Serial monitor with data. I could no longer re-program it after that. The T4 still showed up as COM32, but not as a HID USB input device. My "spare" T4 shows up twice as a HID USB input device with VID 1630 and PID 486 in both cases.
The restore routine (pushing the button for 15 secs) didn't work at all. Getting desperate, decided to short the PROG pin to ground for 15 secs , in case the switch was bad. That actually worked, the red LED flashed momentarily, then after a few more seconds it came back to life. I am now able to reprogram the T4. However, it still does not show up in Device Manager as a HID USB Input device, like the spare T4 does.
I am sure the switch is bad, as I also can't get it to manually enter Program mode by depressing it. The switch is properly positioned, and looks like the pads should have had solder adhered to them, but the contacts/PC pads are basically hidden at this stage.
I'm not sure if I'll run into problems with it not showing up as a HID device.
I thought I'd report this in case there are other bad switches on T4s out there.