5 countries in 4 weeks? That is crazy. At a maximum you would be able to cover 2 countries in 4 weeks, say Germany and the Netherlands. Germany and Spain? Forget it, you'd need at least 6 weeks. For Italy, Switzerland, The Netherlands, Germany and Spain together you'd need approx. 3 months.
If you are mainly interested in Spain or Italy and or have a small budget I suggest you travel with the bike and a small tent. Stay on camping sites and cook your own food on a camping stove. A 125 ccm bike is not very fast, with luggage it will be even slower. Do not try to combine another country with either Italy or Spain. Instead take your time to see the small villages off the beaten path. Tour the whole country.
When you travel by train a tent is extra baggage, plus without your own transport it is difficult to get to camping sites. So when travelling by train you are bound to stay at hostels, which is (combined with the train ticket) more expensive. Also instead of seeing the countryside you'll be pretty much travelling from city to city. Yet you should be able to cover longer distances, especially if you take night trains. With this method combining Switzerland, The Netherlands and Southern and Western Germany might be doable, if a bit rushed.
While it is true that with train+hostel your chances of meeting people are higher, you can also have great fun with bike+camping sites. Bike+Camping just means that you have to make a little more effort, you have to approach the people and try a little harder. (= Sitting alone somewhere hoping for somebody to notice you and talk to you won't work when you are on a campingsite.)
I hope this helps you decide what you want.