The Sinner completed the Season 4 finale not knowing it was the series finale

TVLine

"I never know if the show is getting renewed until the season is shot and edited and broadcast, so I always prepare myself for it to be the end," says creator and showrunner Derek Simonds of Wednesday's series finale. "I did that a bit with Season 3, but Season 4 was a welcome opportunity to fully complete Ambrose’s journey… and I always imagined that Season 4 would close his trajectory in a (hopefully) satisfying way. That was always the intention."