I'd play MH but I'm waiting for MH4 instead. Isn't MH3U pretty much the same experience as the Wii version?
Have you played Tri? If not, MH3U will be totally new to you. If you have, and enjoyed it, that won't lessen the experience in MH3U, in my opinion; it's almost an entirely different game. It's Tri but with at least twice as much content. More weapon classes, armour, monsters (50 large monsters vs. 18 in Tri), quests, etc. Its multi-player experience is also looking to be much better. The ability to communicate via microphone/headset is already a huge addition.
MH3U is a true sequel in all but name.
If you're waiting for MH4, you'll be waiting a while. Assuming it even comes to North America, the Japanese version isn't launching until Summer 2013. North America, therefore, can expect a mid-late-2014 launch if history's any indication. MH3U has been out in Japan since December 2011, for example.
Additionally, MH4 has only been announced for the 3DS so far, which is a sub-optimal platform for the franchise, in my opinion, especially if they snuff online play like they did with the 3DS version of MH3U.