بسم الله الرحمن الرحيم
It is mandatory to recite any other Surah after reciting Surah Fatiha in every rakah of four rakah sunnah salah. Reciting only Surah Fatiha in the last two rakah of four rakah of fardh salah is specific with fardh salah only. However, it should be noted that any other Surah should be recited in such a way that the second rakah is not longer than the first one i.e. one should not read Surah Kauthar in the first rakah and Surah Kafirun in the second rakah, it is makrooh to do so.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.