Is it permissible to combine the Maghrib and Isha prayers after arriving from a journey, if the time for Maghrib had begun while I was at the beginning of the journey and had not yet left the city's buildings, knowing that the journey duration is four hours and the bus might not stop?

If the bus stops, it is better to pray Maghrib on time and not combine it with Isha. However, if it is not possible for the bus to stop, then it is permissible to delay Maghrib prayer and combine it with Isha by delaying the combination (jam' ta'khir) after the journey ends. If the prayer time arrives while on the bus: If it is a prayer that can be combined, such as Maghrib with Isha, then one should intend to combine them and pray them upon disembarking. If it is a prayer that cannot be combined, such as Fajr, and its time will expire before the bus stops, then one should pray it on the bus according to their ability.