The author didn't specify which Mac and external monitor he used but it sounds like he had specific issues that most people probably don't. 1. macOS has bugs r

The author didn't specify which Mac and external monitor he used but it sounds like he had specific issues that most people probably don't. 1. macOS has bugs related to font aliasing from misconfigured setups that makes using external screens harder to use, such as font-aliasing settings that were removed in the latest macOS updates. Some folks had to do a clean macOS install that fixed the problem with Catalina. 2. I wouldn't recommend using HDMI > TB port, DisplayPort is recommended for any 4K setups IMO. He may be using a 30hz screen via HDMI where most older Macs can't do 4k@60hz at all, in this case, even 4k @ 30hz is very uncomfortable to use. 3. If he never had retina screen before, it is possible by moving to a larger screen, he is focusing his eyes more often and causing blurriness. This could be a sign of an eye condition; such as not taking enough breaks and/or vision has degraded. I had this issue and my astigmatism actually got worse, once I got better glasses, no issues. 4. He may need to readjust his desk setup or get a monitor arm that places the monitor correctly to his eye vision. A lot of people used monitor with its stand and they are looking at them facing down, causing more pain than need be.