Exactly :) It's very informal

“Confess” is formal and has many meanings and uses that “fess up” does not have. He confessed his guilt. I confessed my sins. (Not “fessed up”) “Fess up” can’t be used with a direct object, and has a general sense of telling the truth in an informal setting where there isn’t a specific charge. It sounds more natural in the imperative. Father to son: “I know you were in the car when it crashed. What happened? Fess up! “ (Tell the truth, even if it means disclosing your own wrongdoing.) (It would be very unnatural to use “confess” here.)

Yes, exactly.

Yes exactly. It means tell the truth

Yes it’s the same meaning