What are the prayers that can be performed during free time, and what is the authenticity of performing four rak'ahs before 'Isha and Maghrib prayers?

Nawafil (supererogatory prayers) can be performed at any time except during the prohibited times (after Fajr until the sun rises to the height of a spear, after Asr until sunset, and when the sun is at its zenith). The best way to perform absolute nafl is by praying two rak'ahs and then performing the taslim (definition) (salutation), though four rak'ahs with a single taslim are permissible.

Among the nafl prayers are:

Salat al-Tasabih (Prayer of Glorifications): Four rak'ahs, in each rak'ah one recites Al-Fatiha and a surah (definition), then says the glorifications (Subhanallah, Walhamdulillah, Wala ilaha illallah, Wallahu Akbar) 15 times before bowing (ruku'), 10 times during ruku', 10 times after rising from ruku', 10 times during the first prostration (sujud (definition)), 10 times between the two prostrations, 10 times during the second prostration, and 10 times after rising from it (75 glorifications in each rak'ah). It is preferable to perform it daily, weekly, monthly, annually, or at least once in a lifetime. Salat al-Duha (Forenoon Prayer): Its time is from after the sun rises to the height of a spear until shortly before Dhuhr. Qiyam (definition) al-Layl (Night Vigil Prayer): Its time is from after Isha until Fajr. The prayer between Maghrib and Isha.