Okay, I got one K4 running. I just needed to charge the battery awhile and boot to main with MfgTool, then it saw ENABLE_DIAGS and booted back to diags, but not at the all-black diags screen they were both stuck at. Perhaps they both had dead batteries that could not completely boot to diags, but WOULD NOT CHARGE in that mode. I had to change modes with MfgTool just so they could charge.
I opened a diags SSH session on a K4 running 4.0.1 firmware. I installed tcc according to above instructions, compiled the demo, and it is drawing square and round tubes on the eink screen as I type this.
I will try again in main to see if I can duplicate your problem there. I need to install usbnet on it first...
So, my K4 4.0.1 runs tccmake just fine (in diags mode). I will test main boot mode (soon).
UPDATE: Okay, both K4s are working now. Apparently, they drain the battery completely empty then act bricked if you unplug them from USB while they are left in diags mode, and then plugging in the USB does not seem to charge them in that mode. Need MfgTool boot to main to get them where you can charge them with a wall charger... Or something like that. The key is, do not leave them in diags with USB disconnected, for any length of time -- always exit diags for power down.