How will you record the different journeys?
Have you got a way of making sure you have got them all?
Perhaps there is a way you could use the three-station journeys to
help you find all the four-station journeys?
When you're looking for a pattern in the numbers, it might help to
go even simpler as well - how many journeys would there be for just
two stations?
Can you see any way that the numbers of different journeys are
connected to each other? It might help to write them in a list or
table.