A post on IGN says Capcom's reason for the delay is because, "further quality enhancements are needed in order to meet... expectations." The Bond-villainesque ellipses at the end of the sentence is a little concerning, but we're sure that just means they'll be working really hard on it. There's no specific date yet, but the game will be released in Japan in the summer. Because of the delay, and weakened Resident Evil 6 sales, Capcom has lowered its projected profits, but still expects to see a $77 million profit by the financial year's end.
Check out a Monster Hunter 4 trailer from IGN below, featuring the game's larger-than-life action on a 3DS screen.