Yes, I see that phrase occasionally - "Autosomal DNA is only reliable back three or four generations."
At best, it seems to me a misleading statement. Maybe, as "C Handy" suggests, the "reliable" part refers to how likely you are to get a match a particular cousin at a specified level of relationship. If so, that shows a fundamental misunderstanding of how auDNA is used in genealogy.
Matches of at least about 10cM are perfectly "reliable", even if it's a match to a 7th cousin (7C)!
By my count, "4 generations back" gets you to your great-great grandparents; so it's about your 3rd cousins (3C). That's the easy stuff! The further back stuff is where it really gets INTERESTING.
As to "sticky segments", I subscribe to the theory that everybody likely has a handful of these. I've identified four or five so far, for me, and that's just on my father's side! One of them is from too far back for me to even know how I'm related to those dozens of people, but I can tell it's on my gt-gt-gt grandfather's side. If it's not from his maternal grandparents (which would make them 6Cs), then it's even further back.