Is it permissible for a traveler to pray Dhuhr and Asr as four rak'ahs when combining them, and what is the time interval between the two prayers when shortening them? Is it permissible for a traveler to pray Dhuhr as four rak'ahs behind a resident imam and then shorten Asr afterwards?

The Sunnah is to shorten Dhuhr and Asr, and it is permissible to pray them in full. It is also permissible for you to pray Dhuhr behind a resident Imam, and then combine Asr, shortened, after it. However, if you pray behind a resident, you must pray in full.

Many scholars believe that immediate succession (muwalat) between the two prayers is a condition for the validity of combining them, meaning that Asr should be prayed immediately after Dhuhr, unless there is a conventionally brief interval. However, some scholars do not stipulate immediate succession between the two prayers when combining them.