Is it permissible to travel with a mahram who will separate from me four or five hours before the end of the journey out of necessity?

It is not permissible for your mahram (definition) to leave you without escorting you to a country where you can reside, unless he is compelled to do so. If he leaves you, it is not permissible for you to complete the journey without him; rather, you must return with him or stay where he resides. If the mahram dies or disappears, and the remaining distance is short, and completing it is easier and safer than returning, then you should complete it. Other than these cases, it is not permissible for you to complete the journey without a mahram, except in the situations where some scholars have permitted a woman to travel without a mahram.