What is the manner of sitting (whether Iftirash or Tawarruk) in a four-rak'ah prayer if the worshiper caught the Imam in the last rak'ah and only prayed the first rak'ah? And is Tawarruk only in the second or fourth rak'ah?

The latecomer (masbuq) sits in the-tawarruk posture with the Imam during the final tashahhud. However, if he sits in the iftirash posture, there is no harm. As for the tawarruk posture, it is sunnah in the final tashahhud of a four-rak'ah or three-rak'ah prayer, but not in the middle tashahhud, nor in the final tashahhud of a two-rak'ah prayer.