We were recently reminded that 4,1 Mac Pros, even with the 5,1 Firmware, can't natively run updates to 10.13. This can result in being stuck at 17G65 or even the dreaded 17G66 (no Nvidia drivers for 17G66)
The fix is simple, and was provided by DosDude, who has made life easier for Mac users running newer OS's.
Scroll to the bottom of the linked page and you'll see some FAQ's. Right above these he has a section called "Updates". The little script for download will allow you to use Software Update natively to get latest Security Patches. Simply download the script, open a terminal window and drag the script to it. Hit "Return" and you'll be asked for your admin password. Once entered, the script will run, and then ask if you'd like to have it check for updates.
At this point you can close the terminal window and Restart. Once booted back up, you'll be able to run the Security and OS updates normally, as Apple intended. The Nvidia driver updates will allow you to get their newer drivers as well.
Thanks again to DosDude, consider sending him a small donation for making our lives so simple.