Hi Nikolaos, hi nerd
i'll visit the north this summer, too, so I can't tell you about good places to visit. but I collected some interrail-experiance in france last year. we met a lot of other interrailers and once a group of four people.
the rule is:
never travel in a group of 3, 5, 7, 9 and so on. the danger is that one will seperate from group.
and if there are girls with you, you know, suddently they are ill and you have to book a room in a hostel, and it is expansive, and you will lose time, and if you are a big group, it is possible that ever member wants to travel to a different spot... you are more flexible, you are faster when you are just two.
take this advice or not, i know groups that split up and friends became enemies.