4 is the best

I normally travel in a group of four people. The reasons:

1. Long journey trains or high speed trains always have 4-seats cubicle with table in the middle or 4-beds compartment.
2. Quadruple room is always cheaper than double or triple room. Beyond that is dormitory type already, I feel.
4. 1 is lonely; 2 is normally couple; 3 is odd combination; 4 is just nice; beyond 4 is too many.
5. Each person will have a buddy to look after or rely upon.
6. 2 people go to the toilets while the other 2 look after the bags.
7. 2 fall asleep, the other 2 look after safety while chit-chatting over coffee.

Many more reasons but those are the main ones.