Is every prayer after Ishā’ considered a part of Qiyām al-Layl, and is it better to pray two rakahs after Ishā’ and then Witr after Qiyām al-Layl in the last third of the night, or is praying four rakahs after Ishā’ considered a part of Qiyām al-Layl or not?

Not every prayer after Isha is considered part of Qiyam al-Layl (night prayer). The regular Sunnah prayer after Isha is not counted as such, and it is not permissible to combine the intention for both. It is best to delay Qiyam al-Layl until the last third of the night, and it is recommended to begin it with two light rak'ahs. It is permissible to pray any number of supererogatory prayers after Isha, whether they are intended as Qiyam al-Layl or not. It is also permissible to divide Qiyam al-Layl by praying before sleeping and after waking up, and it should be concluded with Witr prayer.