Let's list the possibilities of crime:

Both a and b are innocent.

B is guilty and B is not guilty.

C is innocent and B is guilty.

Both parties are guilty.

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

This sentence includes the first three possibilities of the above crimes:

Both a and b are innocent.

B is guilty and B is not guilty.

C is innocent and B is guilty.

Four witnesses said that the testimony of the third witness was false.

This means that everything about ABC is fake.

There is only one last case:

Both parties are guilty.