Tocilizumab (Tyenne/Actemra) doesn't work completely for everyone. People have different amounts of the inflammatory substances that cause PMR and GCA. Tocilizumab blocks IL-6, which is the one of the main sources of inflammation. Actemra has worked very well for me. I have been off of prednisone for 6 months. But according to Google, Tocilizumab only works completely for 50 percent of people. It sounds like you are part of the other 50 percent, and that's why you need to keep taking prednisone. But I wouldn't think that means you have to keep taking prednisone forever. Here is a quote from Google AI about PMR:

"for many people, Polymyalgia Rheumatica (PMR) goes into remission and symptoms go away within 1 to 3 years, with roughly half of individuals able to stop treatment within 2 years. While 2 years is a common timeline, the duration varies for everyone, and it is not uncommon for it to last longer, sometimes requiring treatment for up to 5 years. "

It sounds like you need to keep taking 4 mg of prednisone for the foreseeable future. Hopefully your PMR will burn out at some point. My rheumatologist has told me that I am half way through a 3 year plan of using Actemra to treat GCA (at a minimum).