Monster Hunter 4 Ultimate will allow for online multiplayer on a handheld platform for the first time, but online is still slightly restricted by region, according to Capcom.
Over on Capcom Unity, Capcom Community Specialist Yuri confirms in a comment that western players will not be able to play online with Japanese players, since the Japanese and western releases of the game will be hosted on different servers.
This is in line with Monster Hunter 3 Ultimate, which—following a patch—allowed American and European players to play with one another, but not with Japan. One could argue that this makes sense, since Monster Hunter is so heavily reliant on cooperative play, and the language barrier between Japan and the West could prove to be a major hindrance.
On the bright side, Yuri confirmed that you will be able to play online with anyone, not just people on your friends list.
Monster Hunter 4 Ultimate will be be released in Japan this Fall. North America and Europe will get the game in early 2015.