How authentic is the Hadith: "Whoever prays four rak'ahs after Isha, and recites therein and perfects their bowing and prostrating, his reward will be like the reward of one who prayed them on Laylat al-Qadr"?

The aforementioned hadith was narrated by Al-Daraqutni with the wording: "Whoever prays four rak'ahs after 'Isha' and recites in them, and perfects their bowing and prostration, his reward will be like the reward of one who prayed them on Laylat al-Qadr." This is a statement of Ka'b, not attributed to the Prophet, peace and blessings be upon him, therefore it is not considered a proof. Other narrations have been reported concerning the فضائل (virtues) of four rak'ahs after 'Isha'.