Can Ipad 4 bought in USA work with other mobile operators in Europe?
In general, yes. Apple has a very easy to read page listing all the LTE channels each iPad supports.

http://www.apple.com/ipad/LTE/

Since no iPad is locked to a carrier, you don't have to jailbreak or unlock them - just pop in the Sim card for whatever carrier you like and you'll be pretty much ready to go. You should also check the non-LTE radio frequencies for each model by looking over the technical specifications page.

http://www.apple.com/ipad/specs/

In the US, the Verizon &amp; Sprint models also speak CDMA as well as operate on all the typical GSM data frequencies - so you might prefer a pure GSM model or one that can work on CDMA if needed, but this won't make much difference for many that want to use an iPad in Europe.