✅ Answer: Several narrations have been mentioned in the books of hadith regarding the third and fourth rak'ahs of prayer. The most famous and widely accepted opinion, which has more hadiths and also contradicts the views of the general public, is that saying the Tasbihat is better. Here are a few of those narrations:

قَالَ وَ قَالَ الرِّضَا إِنَّمَا جُعِلَ الْقِرَاءَةُ فِی الرَّکْعَتَیْنِ الْأَوَّلَتَیْنِ وَ التَّسْبِیحُ فِی الْأَخِیرَتَیْنِ لِلْفَرْقِ بَیْنَ مَا فَرَضَ اللَّهُ مِنْ عِنْدِهِ وَ بَیْنَ مَا فَرَضَهُ رَسُولُ اللَّهِ صلی الله علیه و آله

📚 *Man La Yahduruhu al-Faqih*, Vol. 1, p. 308

Translation: Imam Ali al-Ridha (AS) said: Indeed, in the first two rak'ahs of prayer there is recitation, and in the second two rak'ahs there is Tasbihat, to distinguish between what Allah has made obligatory from Himself and what He has made obligatory from the Messenger of Allah.

بِإِسْنَادِهِ عَنْ مُحَمَّدِ بْنِ عِمْرَانَ فِی حَدِیثٍ أَنَّهُ سَأَلَ أَبَا عَبْدِ اللَّهِ فَقَالَ لِأَیِّ عِلَّةٍ صَارَ التَّسْبِیحُ فِی الرَّکْعَتَیْنِ الْأَخِیرَتَیْنِ أَفْضَلَ مِنَ الْقِرَاءَةِ قَالَ إِنَّمَا صَارَ التَّسْبِیحُ أَفْضَلَ مِنَ الْقِرَاءَةِ فِی الْأَخِیرَتَیْنِ لِأَنَّ النَّبِیَّ ص لَمَّا کَانَ فِی الْأَخِیرَتَیْنِ ذَکَرَ مَا رَأَی مِنْ عَظَمَةِ اللَّهِ عَزَّ وَ جَلَّ فَدَهِشَ فَقَالَ سُبْحَانَ اللَّهِ وَ الْحَمْدُ لِلَّهِ وَ لَا إِلَهَ إِلَّا اللَّهُ وَ اللَّهُ أَکْبَرُ فَلِذَلِکَ صَارَ التَّسْبِیحُ أَفْضَلَ مِنَ الْقِرَاءَةِ

📚 *Man La Yahduruhu al-Faqih*, Vol. 1, p. 309

Translation: In a narration, Muhammad ibn Imran asked Imam Ja'far al-Sadiq (AS): Why is saying the Tasbihat in the second two rak'ahs better than reciting Surah Al-Fatiha? He replied: Because when the Messenger of Allah (PBUH) was in the second two rak'ahs during the Night of Ascension, he recalled the greatness of Allah and became awe-struck, and said:

سُبْحَانَ اللَّهِ وَ الْحَمْدُ لِلَّهِ وَ لَا إِلَهَ إِلَّا اللَّهُ وَ اللَّهُ أَکْبَرُ

So for this reason, saying the Tasbihat in the second two rak'ahs of prayer is better than reciting Al-Fatiha in these two rak'ahs.

مُحَمَّدُ بْنُ عَلِیِّ بْنِ الْحُسَیْنِ بِإِسْنَادِهِ عَنْ زُرَارَةَ عَنْ أَبِی جَعْفَرٍ أَنَّهُ قَالَ لَا تَقْرَأَنَّ فِی الرَّکْعَتَیْنِ الْأَخِیرَتَیْنِ مِنَ الْأَرْبَعِ الرَّکَعَاتِ الْمَفْرُوضَاتِ شَیْئاً إِمَاماً کُنْتَ أَوْ غَیْرَ إِمَامٍ قَالَ قُلْتُ فَمَا أَقُولُ فِیهِمَا فَقَالَ إِذَا کُنْتَ إِمَاماً أَوْ وَحْدَکَ فَقُلْ سُبْحَانَ اللَّهِ وَ الْحَمْدُ لِلَّهِ وَ لَا إِلَهَ إِلَّا اللَّهُ ثَلَاثَ مَرَّاتٍ تُکَمِّلُهُ تِسْعَ تَسْبِیحَاتٍ ثُمَّ تُکَبِّرُ وَ تَرْکَعُ

📚 *Man La Yahduruhu al-Faqih*, Vol. 1, p. 392

Translation: Imam Muhammad al-Baqir (AS) said: Never recite anything in the second two rak'ahs of four rak'ahs (do not recite Surah Al-Fatiha), whether you are the imam or a follower (i.e., the one praying alone or behind the imam). The narrator said: So, what should I say in those two rak'ahs? He replied: When you are the imam or praying alone, say three times:

سُبْحَانَ اللَّهِ وَ الْحَمْدُ لِلَّهِ وَ لَا إِلَهَ إِلَّا اللَّهُ

Until you complete the remembrance and Tasbih, then say Takbir and go to Ruku.

Note 1: In most narrations, it is mentioned that it should be said:

سُبْحَانَ اللَّهِ وَ الْحَمْدُ لِلَّهِ وَ لَا إِلَهَ إِلَّا اللَّهُ وَ اللَّهُ أَکْبَرُ

Note 2: Although in a few hadiths, the permissibility of reciting Surah Al-Fatiha in the third and fourth rak'ahs is mentioned, saying the Tasbihat is preferred according to these narrations.

It is worth mentioning that many scholars of the general public considered reciting Al-Fatiha in the third and fourth rak'ahs of prayer to be obligatory, while others considered it more preferred than saying the Tasbihat.