Ipad shouldn't be a problem generally in Europe; I don't know about greece specifically though. You'll need a sim for every country you'll visit as roaming data charges are huge.
Iphone4 is a bigger issue for you as I'm guessing it's an AT&T locked handset? You'd need to get it unlocked. Depending on where you go, getting hold of a micro sim might be difficult, so you'd probably be best advised to get a sim cutter.
