The legal drama, which premiered in 2015, revolves around Thorton's Billy McBride, a down on his luck lawyer, who ends up taking a case against a big corporation.
The final chapter will see Billy return to his BigLaw roots after Patty (played by Nina Arianda) takes a job at a prestigious white-shoe law firm in San Francisco. Together, they try to take down one of America's most insidious Goliaths: the opioid industry.
''I always hoped that the final season was going to be the best one and in this case, I think they did it. I always signed up to do four seasons because I'm mostly into music and movies,'' the 66-year-old actor, known for films like ''Monster's Ball'', ''Armageddon'' and ''Intolerable Cruelty'', told PTI in a Zoom interview.
''Although I love this idea of the series where you can develop a character over time, I think sometimes a series may go on too long because you have this fanbase who don't want it to end. But, at the same time, you want to go out on a high note as opposed to a low note. I'm not sure we could beat this season with season five. I think it's a perfect way to end it,'' he added.
The subject matter of the eight-episode season, which starts streaming on Amazon Prime Video from Friday, is also very current, said Oscar winner Thornton.
''It deals with pharmaceutical companies. That's in the news a lot these days. But I think the fans will be satisfied with the ending.'' As Billy deals with his own chronic pain in the final season and Patty can't shake the feeling she's being used, their loyalties will be tested, putting their partnership on the line.
''In a world where money can buy anything, even justice, they'll have to risk everything to do what's right,'' reads the official logline of the show.
Once the audience gets to know the characters, they want to see what's going to happen to them next, believes Thornton, who earned his second Golden Globe Award for ''Goliath'' in 2017. His first Golden Globe win was for his portrayal of a sociopathic hitman Lorne Malvo in the FX miniseries ''Fargo''.
''People are naturally curious, that's why they like soap operas. They want to see what's going to happen to Suzy or Brad or whoever it is. Once people start watching something that's going to continue, they become more curious,'' he added.
Thornton said there's something about the underdog story that intrigues people, which is why the character of Billy McBride, who used to be on top at one point of time and is now trying to survive, found a resonance.
''I think everybody sees themselves as the underdog. Everybody wants to succeed. As children we all have dreams of becoming something and when you're young you see yourself as an underdog.
''That's why the underdog has always appealed to the people. It happens in sports too. It's always exciting watching someone who doesn't seem to have a chance of winning.'' Also a musician, filmmaker and writer: Thornton is a multihyphenate.
His late brother Jimmy Don wrote a number of songs and he recorded two of them (''Island Avenue'' and ''Emily'') on his solo albums.
''I grew up in music so that was my first thing. There will always be a special place in my heart for it. With music you get an instant reaction and satisfaction from it,'' Thorton, who is a founder member and vocalist of the American rock band The Boxmasters, fondly said.
''I love acting. I always have,'' he added. ''I love them all equally.'' But writing and directing ''the hardest thing I've ever done'', he said.
Thornton won an Academy Award for best adapted screenplay for writing the 1996 indie drama film ''Sling Blade'', which was his feature directorial debut. He also starred in the movie which earned him an Academy Award nomination for best actor.
''Writing and directing are the ones that I probably have the least interest in. I like writing more than directing. Directing is fun while you're doing it but you have a lot of other stuff to deal with. Plus it takes a year and a half out of your life. So, music and acting will be at the top of the list,'' he said.
He has directed several other films, including ''Daddy and Them'', ''All the Pretty Horses'' and ''Jayne Mansfield's Car''.
The final season of ''Goliath'' also stars Nina Arianda, Tania Raymonde, Diana Hopper, Julie Brister, Bruce Dern, Brandon Scott, Jena Malone, and J K Simmons.
The series hails from Amazon Studios, with executive producers Lawrence Trilling, Geyer Kosinski, Jennifer Ames and Steve Turner.