What is the ruling on one who prays 'Isha' behind one who is praying tarawih?

The questioner : Well, Shaykh, last night one of the brothers came to the mosque and found that the prayer had ended, the 'Isha' prayer. So he came, of course, and waited until the tarawih was established, and he prayed two rak'ahs, of course with the intention of the obligatory prayer. He prayed two rak'ahs, and when they finished the two rak'ahs, he stood up and added two more rak'ahs. Does this mean that this action is prescribed?

The Shaykh : This is what I did yesterday, very recently. This is what I did because I was invited to my brother-in-law Nizam's place, and the prayer caught me at the al-Jabiyah mosque. Do you know al-Jabiyah? You know the mosque there, on the side of the road in a very large open area. Anyway, we stopped the car, and the congregation was coming out of the obligatory prayer, so I actually prayed two rak'ahs behind them while they were praying tarawih. When the imam finished, I stood up and completed the two rak'ahs. Then I caught up with one rak'ah of the first rak'ah of the second witr, and so on. I completed the two rak'ahs that they were in the last of the fourth [rak'ah], and they were raising their voices reciting three times. I completed my witr, and so I continued the prayer with them until the end. This would be considered following the one who is praying a voluntary prayer, and this has passed you by as being permissible.
Yes.