Please solve the case

Both Party A and Party B are thieves. They are accomplices, right?

First of all, what the fourth person said is true, which means that what the third person said is false, which means that at least one of what the first two people said is true. The negative proposition of this sentence is true, and the negative proposition of this sentence is that what the first two people said is false, so Party A and Party B are thieves.