If a traveler joins the Imam during Tarawih prayers with the intention of praying 'Isha, should he pray four rak'ahs or conclude with two?

The traveler prays with the Imam if it aligns with the Sunnah and shortens the prayer, unless the Imam will pray four rak'ahs, in which case the traveler must complete four rak'ahs so as not to differ from him. If the traveler enters the mosque and the Imam is praying Tarawih while the traveler has not yet prayed 'Isha, then he should join them with the intention of 'Isha prayer and make taslim with the Imam if the Imam prays two rak'ahs. If the Imam will pray four rak'ahs, then the traveler should complete it as four. It is permissible to pray a fard (obligatory) prayer behind someone praying a nafilah (supererogatory) prayer, as evidenced by the action of Mu'adh ibn Jabal (may Allah be pleased with him), who used to pray 'Isha with the Prophet (peace be upon him) and then lead his people in the same prayer.