Ugh! Now as I was trying to write, I have the same machine you do and am running macOS 10.13.6 for some time now. I could update to 10.14.6 if I wanted to, but I don't.
The key is to update your 2009 4,1 machine to the 2010-2012 5,1 version which is quite easy to do and doesn't cost any money. Complete instructions can be found in this MacRumors forum: https://tinyurl.com/t6g3w2m