The 1st one was the best. This one isn't close to what the 1st one was like. The machines are by FAR better in the 1st and 2nd movie, and the plot of the 4th movie could have been written by a high school student.
Spoilers: Jigsawa dies and needs someone to continue his legacy, he has an FBI agent doing so using his machines. Several cops come up missing and the main character has his wife taken by the fbi agent and puts him through tests to see if he values his work of saving lives and solving the jigsaw crimes more than his wife. He fails, you dont know what happens to the wife, every one dies and the black guy has a gun shot wound to the chest. The fbi agent was one of the people in the last machine, he got up and walked off and pretty much told the black guy he was lame. end of movie.