I doubt they'll make Wii2. You might say that there is proof that every 5 years they come out with something new, (See pic below) well that will have to be the 3DS for 2011. Theres only a hand full of games that are good on wii and Monster Hunter TRI being one of them. As for PS4 well see below when that will be released. I personally hope it will be on Ps3 or Xbox 360 beacuae well we all know that MH has beaty graphics and a step up into hd would just make you druel. If capcom is smart theyll settle with one or two machines instead of making fans owning everything possible. Getting back to the 3DS, I dont want to sit there with my stylus and kill rathalos! I want to sit in front of my widescreen with a controller ffs.
I dont know when MH4 will come out, probably within the next 2 years. Like most games it may even come out on multiple platforms this time :)
So my guess is...
WII, PS3, PS4 AND OR 360.
http://www.ps4playstation4.com/flash/PS4-CountDown.swf?bg=FFFFFF