I want to know that if I buy a japan ver. 3DS, will I able to play monster hunter 4 online in the US when it release (connect to japan server while I live in USA) ?

Depends, the Wii U is region locked, but they updated it so you could play with people in the PAL region (and maybe Japan, but I dont fully know). There is no hardware reason it will. Ot work as far as I know, so its up to the choice Capcom makes.