I agree with all of the points here, which is to say that there isn't a perfect answer.
Higher HOC (e.g., cutting 4 and above) does mean that more of the grass isn't getting much airflow, and that is a concern for disease development. I cut my fescue at 4.5 during the spring, but I transitioned to 4 at the start of summer. Was that really necessary for disease prevention? I can't say, but it seemed reasonable. I can say that, since I transitioned to 4, my color hasn't been as good.
Many people in the transition zone also use fungicides with their fescue, regardless of HOC.
Do check out GCI Turf on youtube. That's a great resource for fescue, especially fescue in the Greensboro area. FWIW, I believe that he cuts between 4-5 in the summer, partly to maintain color. But he also uses fungicides.