Third party ROMs / firmwares are now available for the Moto Z4. But for that, you will need a custom recovery like TWRP, which we now have. You will need a Windows computer to flash the TWRP.
You will also need basic ADB tools (courtesy of xda forums). Unzip it, this is smaller version of ADB which is good enough for us since we will not be creating applications. Download the TWRP for Moto Z4 from here and unzip it. Then, open a command prompt in your fastboot folder and type in this into the command prompt.
fastboot flash boot_a twrp-foles-test-v4.img
And then, this,
fastboot flash boot_b twrp-foles-test-v4.img
Then, use the volume buttons to boot into recovery. And that is it. You have successfully installed TWRP onto your Moto Z4.
Keep in mind that this is a dirty port of the TWRP and there could some bugs here and there, but it is working as expected.
Another thing to keep in mind is that you will loose warranty once you flash this TWRP onto your Moto Z4, however if you flash the factory ROM over this, it will replace the TWRP with the actual recovery and delete all data as well.