In the USA, 4g lte networks are not so widespread yet. When there is no 4g, your ipad will kick into the 3g network.
As far as I know from what I've read on the subject, deggie is right when he says to avoid ATT international plan. It's expensive!
He's also suggested you use pay as you go plans that are hooked up to where you are going. Most countries have their own carriers, so you should research ahead of time online and at bookstores (Frommer's usually has updated info on these subjects - 4g to 3g, and would 3g sim card suffice, and local plans in each area of each country.) When you get to France and then Italy, go to stores you have researched that can give you a more local plan where your rates will be cheaper. Avoid buying at airports or hotels. They usually charge more for everthing. In bigger cities, they're probably all over the place like Verizon, ATT, etc. I have no idea if you can have the cards shipped to you before you go.
Peter Greenberg (sometimes seen on Today show and the View) is a travel expert and has many books out regarding travel. Petergreenberg.com. I'd check him out, too. Google, google, google and that could be a great help!
Though you did not ask, I assume you know about outlet adaptors (if not check out travel smart by Conair - in Best Buys, tech stores, and even drug stores. Travel Smart has a lightweight (about 2.5" cube) all in one - US/Aust, Europe, and UK. Ours also has a usb so ipods can be charged!
I'm sure someone very knowledgeable on the subject might see your question and possibly know exactly what you should buy.
Au Revoir and Ciao!