Is the prayer of a traveler valid if he intended to stay for a simple purpose not exceeding four days, but he shortened and combined the Maghrib and 'Isha prayers?

If you are a traveler arriving in a country and you do not know whether you will stay there for four days or not, then you remain under the ruling of a traveler according to the majority of scholars. What you did by shortening the 'Isha prayer and combining it with Maghrib is permissible, and it does not invalidate your prayer.