Okay, you go a fair amount. Anything trigger having to go? For me, a cup of coffee can do it, anything sugary can do it. So, you'd want to try to go before the bus ride and not have the things that trigger a bowel movement for most people. And go ahead and try to hold it. I can usually forget about it if I have to go unless I am sick. How often do you have to go on these bus journeys? 4 hours both ways in a day is 8 hours travel time, that's too much really. I'd try not to do that very often. And also, if you fear that you will have to go, you begin to have anxiety about it and that compounds your feeling like you might need to go. Then it is mentally related if that makes sense. So, try not to worry as much about it.
Irritable bowel doesn't always mean you have to have frequent bowel movements. How many times a day do you typically go?