Reasoning:

After investigation and study, it has been confirmed that the fourth witness is telling the truth.

Then we know that at least one of the first two testimonies said by the third person is true! "It's fake. Then we know the truth is that the first two testimonies are false.

The first man said, "I only know that A didn't steal it!" " Then there is theft;

The second witness said, "I only know that B didn't steal it!" " "Then B steals.

So A and B are thieves.

Number.

The first witness said, "I only know that A didn't steal it." ( 1)

The second witness said, "I only know that B didn't steal it." (2)

The third witness said, "At least one of the testimony of the first two people is true." (3)

The fourth witness said, "I'm sure the testimony of the third witness is false." (4)

Because (4) is true.

So (3) is false.

So (1) and (2) are both false.

So a and b both stole it.