1

way to travel from Barcelona to Rome - Ryan air starts for 19€ per Pax, one way.

2

have you read this?
http://www.lonelyplanet.com/thorntree/thread.jspa?threadID=2153476

3

Some suggestions for you if you want answers to questions.

This is not useful if you have practical questions about transport. Sure, you may be in four countries, but what's closer to the truth is that you'll be visiting specific places which just happen to be in 4 different countries.
Without more detail, what can anyone say?

A month translates into 1 week 'per country', all the more reason to be more precise with your plans. In particular:

It sounds like a nice plan if you don't take it any further than that. But start thinking about what you'll be doing on a day-to-day basis. You will quickly discover that a month isn't a very long time.

OK, you're new to travel in European countries. But how do you expect to make practical decisions if you have no basic knowledge about getting around? Yes, it's possible to take trains from city to city. Local trains stop at smaller towns. If a train doesn't go somewhere, a bus probably will.
There's a website .. man in seat 61 .. http://www.seat61.com/
Browse it for the basics.

The one practical alternative to surface travel is already mentioned above. If your plan includes big jumps between cities (none of that visiting towns in between) then consider booking a flight on a budget airline. Advance booking for best deals. But you need a plan before you can decide what to do.

4

You're propably not aware of it, but Europe is propably the continent in the world with the best public transport. You can hop on a train in any city and end up in any other (except for the really small towns) for a reasonable price. So if you ask me, don't even bother with a car or a plane. Cars are going to drive you crazy on European streets (and parking is expensive) and planes are just plain boring (and often slower than the train - yes they are).

The other question is why Spain is part of your trip. It seems a bit out of the way. So is there any special reason you want to visit Spain? If not, I would suggest to cut it out and spend a bit more time in any of the other countries, especially France and Italy. England, France and Italy are very diverse experiences, so you really have enough to visit in one month. Between those three, you should have no problems coming around by train.

5

be sensible, OK?
"Often" is 100% irrelevant. What matters is the travel distance and whether or not your're comparing high-speed or normal trains to the flight.

6

I think you should specify what places do you plan to visit in each country and what are your interests, what are you looking for. Otherwise, we can't say if a pass will worth it (although, at the end, in order to know it, you should do the maths comparing the cost of the pass with the cost of point to point tickets), if you'll have enough time to stop along the route when going from one city to another...

7

Aside from flying and railing, there is also sailing. For the trip from Barcelona to Rome, you could use the ferry. (I fact the ferry goes to Civitavecchia, close to Rome) See Grimaldi Lines. The ferry stops on its way in Porto Torres on the NW-point of Sardinia. You could got off there , look around the area and continue on one of the following days.