I got Mojave working on a Mac Pro 4,1, Geforce 710, and a 1 tB fusion drive

[tu2thepoo]

Just throwing a post in here in case people don't find their way to the Mac Pro forum.

First off, I got my Mac Pro prepped per @tsialex 's fine instructions, found here.

Firmware: Got my firmware updated. Read the thread above and follow directions exactly.

Graphics: Removed incompatible graphics cards - in my case, the original Radeon 4870 and Geforce GT120 are not supported, so no go. My Geforce 1080 has no webdrivers (yet) so I bought a cheap Geforce 710: details here. Other options include Radeon 560, Radeon 580, Radeon Vega 56, Geforce 680... all of which were more money than I wanted to spend.

Fusion Drive: I used an old 240gb SSD and a 750gb HD I pulled from an old Macbook Pro 2012. I followed apples instructions: https://support.apple.com/en-us/HT207584
A few notes:

• Wipe (erase) both drives using Disk Utility before doing anything. I had an old cloned copy of MacOS on the HD and it kept failing the unmount/create stage.
• The process also works if you use a bootable USB flash installer (no recovery mode needed, which is important for 4,1/5,1 mac pros that lack Internet Recovery)
• The OWC 2.5" sleds work really nice and I also got a cheaper adapter that lets you keep the original Mac Pro sled.

I installed the Geforce 710, removed all other drives besides the 240gb SSD+750gb HD, booted from my USB installer, created the fusion drive, installed MacOS High Sierra 10.13.6, then used the Mojave full installer to upgrade to 10.14.1 (Use @dosdude1 's utility to download the installer, but please note you do NOT need to patch.)

Screenshots below. Hope this helps anyone searching through the forum!