or.....you know a 4gb card, idk about MH4 but i know smash bros had a similar issue, it only seemed to work with over 1gb free space on the SD card, so a 2GB card would work, but only if you didn't have emunand set up as setting up emunand on a 2gb card means you will have under 1gb usable space left....idk, whatever, but if you rules out a bad rom, try a bigger SD card in the 3DS