What is the ruling on the prayer of one who did not follow the Imam who stood up for a fourth rak'ah, then it became clear that he had cancelled the third rak'ah because he forgot to recite Al-Fatihah in it, and he was not forgetful?

What you did is correct, because your rak'ah (unit of prayer) was valid, and it is not permissible for you to intentionally add to the prayer. As for the Imam, he is excused for performing another rak'ah in place of the rak'ah in which he omitted the Fatihah, because reciting the Fatihah in every rak'ah is a pillar without which the prayer is invalid. If the prayer of the ma'mum (follower) is valid despite the invalidity of the Imam's prayer (e.g., if he was in a state of ritual impurity), then the rak'ah that the Imam invalidated is even more valid for the ma'mum. The scholar Ibn 'Uthaymeen issued a fatwa stating that the ma'mum should not follow the Imam in this extra rak'ah, but rather should sit for the tashahhud and wait to say the taslim with his Imam.