4 weeks based in one town?

My favorite is Siena. Better buses then trains. Big but not too big. To me it's like a small town

You might look into Assisi. I stayed a little over a week there this summer. It was heaven. The train station is down the hill. Easily accessible by bus or by a peaceful (if longish) walk. For me, it had plenty of great sights, wonderful people, and great restaurants. There are also lots of great towns nearby.

Sharon

Green with envy here. Lucky you!

Here's a resource that might be helpful. Rail Map of Italy

If you zoom in all the way, it will show the towns on the various rail lines and you can use the window on the left to find out travel times between places.

Personally, I would recommend Orvieto. It's right on a major rail line so easy access to lots of places (inc. Rome and Florence), and it's a charming hilltop town that's not completely overwhelmed by tourism. Have you been there? Here's a link to some photos and video clips I took in May 2011, in case you're interested.

Arezzo would be another place to consider, though it's not exactly hilltop.

As for Foligno, I was underwhelmed by it when we had a two hour layover between trains in 2011.

Another resource that might be helpful is Rome2rio.com.

I vote for Orvieto too. Train is at the foot of the funicular, and it's just over an hour into Rome Termini. You can be in Siena via car in about 1 1/2 hrs. Many other hilltowns are relatively close.

And a large lake is just west of there. Civita's also just southwest of there.

There are a large number of good accommodations in Orvieto--historic, modern and B&B's.

All 4 weeks in one place? That's quite a bit.
In any case, what should drive your decision is what places/towns you want to visit.
If your target destinations for day trips are in the Siena province, then staying in Foligno wouldn't be a good choice. In that case you should stay around Siena. If your intended destinations are mostly in Piedmont and Lombardy, staying in a town in Sicily wouldn't be good either.
So first list towns/areas you want to visit, then we can help with the best possible "transit hub" that meets your preferences of not being a large city.

Foligno is somewhat industrial and only served by regional trains. How about Fiesole, just outside of Firenze? Small town with )bus) access to small city and high speed rail. Vicenza or Verona are nice, but a bit far north, so depends on what you want to experience. What time of the year?

Fiesole would be a good choice. I find that most Italian big cities are really a collection of smaller neighborhoods, so I wouldn't rule out Florence or some other large city.

If you are interested in Umbria, Spello is a few kilometers outside of Assisi and as far as I know any train to/from Assisi slso stops there. Smaller, fewer tourists. I love Todi but daytrips would require a car (no train station, weird bus schedules). There are Italian language courses offered there as well.

Orvieto is one of my favorite cities, with better bus/train connectios (except on Sundays).

Spello, mentioned by Zoe, is one of the cutest towns! We love it! The train station is a 20 minute walk from teh top of town, or there's a bus. It's very close to Assisi, Perugia, Spoleto. You can get a lot of trains out of Perugia.

I seriously would not rule out Florence -- you can get the neighborhood feel by staying on the "other" side of the Arno. You won't hear much English, and you simply can't beat the transportation. Forgive my spelling, but the district is "Oltrarno." Four weeks in one area does seem a little long to me, but to each his own.

Pistoia...on main train line between Florence and Lucca
Montecatini Terme and Alto...cute funiculare, Poggibonsi access to San Gimignano.
Pisa, Fiesole, La Spezia. all within an hour.

Perugia...Cortona, Arezzo, Assisi, Detura, Todi, Spello, Orvieto, Gubbino, Montevachi
Everything within 1 1/2-2 hrs.

Have fun deciding

By all means, choose Florence! My wife and I have done it twice, once in the fall of 2008 and again this May. Despite staying those two months and other shorter visits, we never ran out of places to see. If you like Italian art, history, and culture you won't either and many of them are free or inexpensive. We stayed in the area around Via Tornabouni, a great area and a short walk to SMN train station or to the car rental spots on Borgo Ognissanti. There is a large selection of one month rentals in Florence, often at substantial discounts over shorter stays, and several local agencies offer them. The center of Florence is really pretty small, easily walkable. We made it even smaller by renting bikes, for under 100 euro each for a month, since the center is now traffic restricted. Get out of the tourist corridor from the Ponte Vecchio up past the Duomo and you'll find plenty of locals, and uncrowded spots. As others have mentioned, florence is a great base for visiting other sights nearby. Or not so nearby, we did a day trip to Rome, only 90 minutes by train. Enjoy your trip, I've certainly enjoyed doing my first post!