Must a traveler repeat the Maghrib and Isha prayers if he combined them with Dhuhr and Asr out of ignorance during the time of Dhuhr, and then completed his journey, and if repetition is obligatory, should it be shortened?

The traveler must repeat the Maghrib and 'Isha prayers because he prayed them before their prescribed time. He must also make up for them in the same manner they were missed, meaning performing the first two rak'ahs audibly. It is more prudent for him to complete the 'Isha prayer to avoid any difference of opinion.