Is it permissible to combine the Maghrib and Isha prayers upon reaching the intended city, after the Muslim had performed the Asr prayer at a resting place during his journey, knowing that he had performed the Friday prayer in the first city? And what is the duration for combining prayers for a non-resident?

What you did is correct if you prayed 'Isha four rak'ahs, because upon reaching one's country of residence, the travel and its concessions cease. It is obligatory to pray Maghrib before 'Isha according to many scholars, and there is no objection to combining them while completing 'Isha. It is not permissible to delay Maghrib beyond its time except with the intention of combining it with 'Isha.