✅Answer: Refer to the following narrations:

1 عَنْ عَبْدِ اللَّهِ بْنِ سِنَانٍ عَنْ أَبِي عَبْدِ اللَّهِ قَالَ: إِذَا كُنْتَ لَا تَدْرِي أَرْبَعاً صَلَّيْتَ أَمْ خَمْساً فَاسْجُدْ سَجْدَتَيِ السَّهْوِ بَعْدَ تَسْلِيمِكَ ثُمَّ سَلِّمْ بَعْدَهُمَا

📚 Al-Kafi, Vol.3, p.355

Translation: Imam Sadiq (peace be upon him) said: If you do not know whether you have prayed four or five units, then after giving the final salam, perform two prostrations of forgetfulness (sujood al-sahw), and after those two prostrations, give the salam again.

2 عَنْ حَمَّادِ بْنِ عُثْمَانَ عَنْ عُبَيْدِ اللَّهِ بْنِ عَلِيٍّ الْحَلَبِيِّ عَنْ أَبِي عَبْدِ اللَّهِ قَالَ: إِذَا لَمْ تَدْرِ أَرْبَعاً صَلَّيْتَ أَمْ خَمْساً أَمْ نَقَصْتَ أَمْ زِدْتَ فَتَشَهَّدْ وَ سَلِّمْ وَ اسْجُدْ سَجْدَتَيْنِ بِغَيْرِ رُكُوعٍ وَ لَا قِرَاءَةٍ يَتَشَهَّدُ فِيهِمَا تَشَهُّداً خَفِيفاً

📚 Al-Tahdhib, Vol.2, p.196

📚 Al-Faqih, Vol.1, p.350

Translation: Imam Sadiq (peace be upon him) said: When you do not know whether you have prayed four or five units, or whether you have shortened or extended the prayer, then recite the Tashahhud, give the salam, perform two prostrations without bowing or recitation, and recite a brief Tashahhud in them.

✅From the two narrations above, it is clear that in the case of doubting between four and five units, the person should give the salam, perform two prostrations of forgetfulness (sujood al-sahw), recite a light Tashahhud, and then give the salam.

Doubt between four and five

Prayer doubts doubts