To put it in the simplest terms, Yowamushi Pedal: Glory Line is a perfect example of a show that has overstayed its welcome.
New Generation (s3) was one thing, but a fourth season for a show where there are lots of new characters (first years) and almost zero time dedicated to focusing on the main character himself is very aggravating, particularly when next to none of the new characters are even likeable.
Throughout my time watching Glory Line this winter season, I often found myself wandering off to another tab to do something else instead. Onoda and his team already won once, and the characters
that struck a chord with me emotionally aren't there anymore. It's boring. As a whole, New Generation and Glory Line both felt unnecessary to me-- somehow or other, Sohoku will win again, right? I don't actually know. But regardless of whether they do or not, it feels like they're dragging out something that really shouldn't have been there from the start.
The story itself is also less compelling now that Sohoku is no longer an underdog trying to win, but rather a champion trying to defend a title. As cliche as it is, more people relate to having to work hard to reach the top rather than already being on the top and having to try and stay there. Toying with whether or not they stay winners isn't nearly as much of a fun journey as wondering how they will win for the first time.
All personal gripes aside, the art, animation, and sound are well done. Objectively, Glory Line is a good show, but when you've seen the previous three seasons, you really wonder if you even want to watch this one, and if they're ever just going to let the series rest.