DJ Thumper, Ethan Allen here started the 3rd listing you referenced. After update to 1.6.0 Prime4 ENGINE will hang on ENGINE OS screen and not boot completely
So, I have a Prime 4 and how I was able to get my Prime 4 in to update mode is: after trying 3 - 4 times to push power button - starts Prime 4 and gets stuck on Engine OS screen. I would HOLD power button down until the machine turned off. Wait 1 minute and then power on again. Finally, it would start properly. I then would attach my computer to the Prime 4 - then put the Prime4 in to update mode. Then choose the v. 1.5.2 file and roll back the v. 1.6 update. I have done this twice and it worked both times. I have Engine OS 1.5.2 mac version if needed. Sorry, no windows version. Hope this gives some resolve. It may take a few more than 3 tries to have it properly loa properly. I think the key is NOT to have your computer connected when initially trying to boot up the machine.