The forged evidence in 4-1 was necessary. He realized he couldn't prove there was a 4th person unless that card was in the court record, since Kristoph took the real one anyway. He probably knew it would be super difficult for Kristoph to actually prove it was a fake, as only Phoenix and Kristoph knew that. He would've taken responsibility if it actually had come to that. That card was the decisive evidence needed to successfully convict Kristoph. There was a very large chance that either Olga or Phoenix would be convicted of murder if that card wasn't there. He wasn't proud of it (and deeply disappointed in himself), but if you were in his shoes, what would you have done? Make a forgery of a piece of evidence that really did exist and convict the real killer, or let the real killer run free without any sort of penalty, and have someone else thrown into the slammer for it? Terry Fawles was a sad case in itself. Being put on death row for a killing he did not commit. Would you like a repeat of a unjust verdict with fries?

That card was necessary, or else Kristoph would be getting away with it, and someone else would've taken the blame.