<< What is the cheapest means of transport to get there >>

There is no way that anyone but you can answer this question. You need to compare pricing for train, bus, airline. Only you know when you will travel and travel dates partly determine pricing; generally the less expensive fares are the farthest from the travel dates.

Without knowing your budget and where you'll be staying in Tuscany this is also almost an impossible question to answer. People can advise on all sorts of accommodations but it the rates are E400 per night and you want to spend E100 per night what is the point of posting recommendations? If you want to stay in Siena and people are posting accommodations for Pienza this is also pointless.

I'd advise that the best answers are derived when posters give some information about their trips.

Easy Jet from Paris Orly to Rome. I don't think you have time to do both Rome and Tuscany. I would pick one.

I agree with kybourbon. Pick one. Eastjet also flies ORY-Pisa.

Does 'visit Italy(Rome and Tuscany) for 4 days' mean 2 days in Rome and two in Tuscany (where?) not considering transportation?
Because even if among Florence and Rome there are now just 1 hour and a half thanks to the high speed train, one afternoon is already gone.
And I said Florence, because if you think to stay in the countryside it will take even more.
Perhaps some more detailed info would help to answer your question.