If I was on travel and came home late in the afternoon and was not able to pray Duhur, Is it allowed or required for me to pray the duhur in 4 rakaats before praying ashar.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu 'alyhi wa sallam, is His slave and Messenger.

When a traveler becomes a resident (reaches his destination or comes back from his travel), he is obliged to perform the prayer at its prescribed fixed time without shortening it. Therefore, if he arrives before the 'Asr prayer, he is obliged to pray Thuhr as 4 Rak'ahs (units) before the beginning of the time of 'Asr prayer, because the permissions which were allowed to him as a traveler, like shortening and combining the prayer, have come to an end. However, since your situation is as you mentioned, some scholars permitted combining the Thuhr and 'Asr prayers, and the Maghrib and 'Ishaa’ prayers, when one is local [not a traveler] due to a dire need or hardship. Nonetheless, in this case one should combine the prayers without shortening them, because shortening the prayers is only permitted during travel.

In such a case, it is better to perform the first prayer at its ending time, and the second prayer at its beginning time, which means that they are combined in form but not in reality [as every prayer is performed at its prescribed fixed time]. 

If a traveler arrives after the 'Asr time has started while he has not yet performed the Thuhr prayer, then in order to be on the safe side and avoid the difference of opinion, it is better for him to perform his prayer in full without shortening it.

Allaah Knows best.