The Terminal command is
defaults -currentHost write -g AppleFontSmoothing -int
Then add the number in one space after the "-int", you can choose between 0 for NO Text smoothing or 1, 2, or 3 for higher levels of Text smoothing. In my opinion 0 works well for Retina Macs and 4K external screens, while 1 is the sweet spot in my opinion, you can try them all to see which works best for you. 3 is the default font smoothing level if you want to go back to the default setting.
Audio Recording, Mixing, and Mastering tutorials for everyone from the home recording engineer to the most seasoned veterans. Enjoy!