Wi-Fi
All products supporting Wi-Fi can communicate with each other using standards such as IEEE 802.11a, b, g, n.
4G
The Americans and Canadians are operating their 4G predominantly in the 700MHz and 2100MHz spectrums, which Apple Ipad 4G is limited to.
Europe is using 800MHz, 1800MHz and 2600MHz - meaning that the New Apple iPad simply will not pick up the European 4G networks. It will still function on 3G and 2G, as well as HSPA+ (colloquially referred to as 3.5G).