1st life: Amanda - John knew that Jeff would kill Amanda after he sees her shoot Lynn. This is the whole purpose of the movie. Jeff is such a vengeful man, anyone could predict that he would kill Amanda after he sees her kill his wife.
2nd life: John - John knew that Jeff would kill him without Lynn there to stop him.
3rd life: Lynn - Obviously, Amanda has just shot her, but also because when John dies, she dies.
Possible 4th life: Jeff - John predicted that with John dying, Jeff would end up dead in his next game. This ends up being true.
Possible 4th life: Corbett - Explains itself.

It would have been interesting to find out if John had survived, would he have let Jeff live after killing Amanda? To follow Jigsaw's rules to the letter, he despises murderers, even before Jeff killed John he had already established that he hadn't learnt anything from his ordeal and still seeking revenge.

Yeah it's confusing.
Now I'm thinking about it, I think him saying four could be interpreted as..
One, Lynn
Two, Jeff
Three, The daughter
And the forth being Amanda. Not in the sence that jeff would kill her, but because she has destroyed her own life by failing the test. She couldn't keep lyn alive, after putting the prison guy in the chains and the detective lady in the angel trap, both of which were designed to be inescapable, so neither victim had a chance to live. The whole movie was Amanda's test not Lyn's. And in failing the test, she destroyed four lives - being Lyn, Jeff, the daughter and herself! It kind makes sence lol

I think John was maybe including himself in the four people's lives being destroyed. To be more accurate, if John was referring to himself, as well as Lynn, Jeff, Lynn and Jeff's daughter and Amanda, it would actually be five people's lives destroyed, due to Amanda's actions in killing Lynn Denlon.