There is a slight difference between Sunnah Ghair Muakkadah and Nafl, with Sunnah Ghair Muakkadah being slightly more emphasised than Nafl. However, this difference is subtle and practically speaking the two are almost the same. Therefore, little focus is given to differentiating between them and they are used interchangeably by classical scholars.

So it is correct to say Sunnah Ghair Muakkadah and Nafl are used interchangeably in the Hanafi Madhab. Consider the following Nass (text).

(وأربع قبل العشاء)

وهذه أيضاً غير مؤكدة لما قلنا، ولهذا كان مستحباً

And four Rakah before Isha. This is also (Sunnah) Ghair Muakkadah, therefore they are Mustahab. (Imam al-Ayni, Sharh Tuhfatu al-Mulook).

In the above Nass, Imam al-Ayni referred to the four Rakahs before Isha as Ghair Muakkadah, but did not hesitate in also referring to them as Mustahab. Many of our scholars referred to them as Nafl or Mandoob and not strictly as Sunnah. This is due to the near synonymous relationship between Sunnah Ghair Muakkadah and Nafl.

     (وَيُسْتَحَبُّ أَرْبَعٌ قَبْلَ الْعَصْرِ، وَقَبْلَ الْعِشَاءِ وَبَعْدَهَا بِتَسْلِيمَةٍ)

وَإِنْ شَاءَ رَكْعَتَيْنِ. (الدر المختار شرح تنوير الأبصار)

And four Rakah before Asr is Mustahab (recommended), and likewise before Isha and after it, with a single set of Salaams. And if he wishes he can pray two Rakahs (Imam Ala al-Deen al-Haskafi, al-Dur al-Mukhtaar).

(قَوْلُهُ: وَأَرْبَعًا قَبْلَ الْعِشَاءِ)

وَهُنَّ مُسْتَحَبَّاتٌ. (الجوهرة النيرة)

And his saying four Rakah before Isha, (I say) they are Mustahab. (Imam al-Haddadi, al-Jowharah).

وَلَمْ يَذْكُرْ الْأَرْبَعَ قَبْلَ الْعِشَاءِ فَلِهَذَا كَانَ مُسْتَحَبًّا

And he did not mention the four before Isha, therefore they are Mustahab. (Imam al-Marghinani, al-Hidaayah).

وندب أربع قبل العشاء. (مراقي الفلاح).

And four Rakah before Isha are Mandoob (recommended). (Imam al-Shurunbulali, Maraaqi al-Falah).

وَأَمَّا مَا قَبْلَ الْعِشَاءِ فَمَنْدُوبٌ لَا يُقْضَى أَصْلًا. (الدر المختار شرح تنوير الأبصار).

As for the four Rakah before Isha, they are Mandoob. Therefore, they are never made up if missed (Qadaa). (Imam Ala al-Deen al-Haskafi, al-Dur al-Mukhtaar).

And Allah (Most High) Knows Best.

– Answered by Shaykh Noorud-deen Rashid (17.09.2021)

See also:

What is the Daleel for the four Sunnah before Fard of Isha?

Is it sufficient to learn the Deen through Q&A only?

See also (video):