Are the 'Isha prayer and the Tarawih prayer valid in the mentioned case, and is it obligatory to complete the Tarawih prayer in sets of four?

Firstly: It is permissible to pray 'Isha behind the Tarawih Imam, and this is the view of the Shafi'is and a narration from Ahmad.

Secondly: If the Imam says the salām, and then stands to complete his prayer, he becomes a solitary worshipper. It is preferable for him to complete it alone. There is a difference of opinion regarding whether he should follow the Imam again for his remaining two rak'ahs, but the most correct view is that it is valid.

Thirdly: Praying four rak'ahs of Tarawih with a single taslim is disliked by the majority of scholars and considered forbidden by some. However, it is valid to pray behind an Imam who does this, following the Madhhab of Abu Hanifa.

Fourthly: There is no harm in praying fourteen rak'ahs of Tarawih. It is preferable to come early to catch the 'Isha prayer in congregation with the Imam and to pray Tarawih from its beginning.