Is seeing a video and the phone owner's confession of adultery considered sufficient evidence to be convinced that the woman is an adulteress, and is this conviction considered slandering chaste women due to the absence of four witnesses?

Four witnesses are required to prove adultery (zina) only due to the gravity of its sin and the serious consequences it entails. Accusing a Muslim of it without conclusive evidence is one of the greatest major sins.

What was mentioned about seeing the woman with the man in bed and her body being covered does not prove adultery, especially with the possibility of alteration through montage and editing.

It is not permissible to believe the man who claimed he committed adultery with her, because he is a transgressor for filming what is impermissible, and for admitting and openly declaring adultery. He might also be lying.

It is not permissible to harbor ill thoughts about that woman without proof, because suspicion is the most mendacious of speech. And looking at pictures that arouse lust is forbidden.

Your conviction that she is an adulteress is not considered an accusation (qadhf) unless you voice it. However, it is an ill thought that is impermissible without proof, and the picture is not sufficient to establish that.