Don’t skip season 1. There’s great character building in there. It does really set actual plot events in motion as well. I don’t know why anyone would say to skip it.
Now, you can decide whether to watch The Gathering (pilot episode, later released as a movie) before season 1 or not. They’ll make references to it in the show, but I don’t think most of the audience had seen it at the time those episodes aired, so skipping it might give you to “correct” experience. Some things are different that you’ll have to overlook, but there’s legitimate plot value there.
Stopping after season 4 is an ok thing to do, but I say go ahead and watch it. (Watch the Season 5 finale even if you do stop there, though).
Between seasons 4 and 5, you can watch or not watch the In The Beginning and Thirdspace movies. That’s the order they aired in. “in the beginning” is a prequel of sorts that adds some color to things we see flashbacks to throughout the show. Thirdspace was a season 4 episode that got cut when they rushed to finish the series early, I think?
The other movies…. Meh. The recent animated one was better than I expected, but I wasn’t expecting much…