I recently inherited an 8-core 4,1, slapped in a blank, reformatted hard drive, installed Mountain Lion (10.8.5) from a USB key, and was able to flash it with no problems. Identified as a 5,1 in ML's System Information. Just had to make sure I had the EFIUpdate mounted and was connected to the Internet. Also lucked out in that it seems to have an EVGA 680 flashed with an EFI ROM, or it could be one of the rare actual EVGA 680 Mac cards that were sold at retail. Sheesh, two PCI power cords? Didn't think it needed that much juice. Right after that, I booted to a High Sierra USB installer. Ran Disk Utility, wiped the drive blank, then ran the HS installer. It told me it would have to do a firmware update (yet again!) so I followed the onscreen instructions, shut it down, held the power button.. watched the DVD drive open, watched the firmware update thermometer. A minute or so later, the machine rebooted again and happily installed High Sierra with Supplemental Update. I looked at the log and it didn't do any HFS to APFS, the machine booted from an HFS+ volume as usual, as it should if it's a platter hard drive. I thought it was kind of weird that it literally ran the firmware update from a wiped drive, probably the firmware updater is written onto the recovery partition or something? HS actually boots fairly quickly on a decent platter hard drive if you have decent RAM (12 GB RAM in triple interlace config, 500 GB WD drive @7200 RPM, 16MB cache) Thinking about getting a Lycom DT120 and a Samsung 960 EVO and see if that will boot now..