Earthbound creator Shigesato Itoi doesn't think there will ever be a Mother 4. He says that there's little chance the game will be developed, as there are no plans to do so.
Speaking plainly, Shigesato-san said that it was "impossible" for Mother 4 to be made. He made his remarks in response to a tweet from a Japanese fan asking him about the possibility of it happening.
ç„¡ç†ã€‚@whirluigi 糸井ã•ã‚“ã«MOTHER4を作らã›ã‚‹æ–¹æ³•ã‚’æ•™ãˆã¦ä¸‹ã•ã„。
— 糸井 é‡é‡Œ (@itoi_shigesato) April 29, 2013
Shigesato-san did not provide any reasoning as to why it would be impossible, but given the source, it's pretty safe to say that it'll never happen.
Fans will have to tide themselves over with the re-release of Mother 2: Earthbound this year on the Wii U Virtual Console later this year.