Can someone please explain the fourth term fallacy to me? I'm trying to understand it by reading online articles on it, but it makes no sense.

https://www.google.com/search?q=fourth+term+fallacy&oq=fourth+term+fallacy&aqs=chrome..69i57j69i60.4198j0j7&sourceid=chrome&ie=UTF-8

Read lots.
I've never heard of this, so can't explain it, but there seem to be some good webpages in those search results.