Is it valid to pray four rak'ahs for the 'Isha prayer in congregation after shortening it to two rak'ahs while traveling?

What you did by repeating the prayer with the congregation is correct and sunnah. If someone prays alone and then finds a congregation praying, it is sunnah for him to pray with them, and it will be considered a supererogatory prayer for him. Scholars based this on the hadith of Yazid bin Al-Aswad Al-Amiri and on the action of Ibn Umar. Among the sayings of scholars regarding this is: If a man prays alone and then catches the congregation, he should repeat all the prayers with the congregation. As for the hadith: "Do not pray a prayer twice in one day," its meaning is that it is not obligatory to pray twice in one day, and it does not contradict the desirability of repeating it.