Is it permissible to combine fasting and shortening prayers during travel, without specifying the duration of stay?

If you intend to reside in a country for four days or more, the ruling of travel is lifted from you. You must complete the prayer, and you are not allowed to break your fast.

However, if you do not know when you will depart, and you have not intended to reside for four days or more, but it is possible that you may travel before that period ends, then your ruling is that of a traveler.

Likewise, if you intend to reside for less than four days, you are permitted to avail yourself of the concessions of travel, and there is no necessary correlation between these concessions.