Is it permissible to shorten the Asr prayer while traveling—combining it with Dhuhr—if the worshiper completed the Dhuhr prayer with four rak'ahs with the mosque congregation?

Each of the two combined prayers is an independent act of worship. Since combining prayers is permissible for travelers, there is no impediment for a traveler to pray one of the combined prayers in full and shorten the other. If one prays Dhuhr behind an Imam who is praying in full, it is obligatory for him to pray in full. Then, he may pray Asr shortened. It does not matter if he was a follower in the first prayer and prayed alone in the second, or if he followed one Imam in the first and another in the second. As Sheikh Al-Uthaymeen mentioned, it is possible for a traveler to pray Dhuhr four rak'ahs and then combine Asr with two rak'ahs. It is a condition that the travel permits concessions and that the other conditions for combining prayers are met.