If the announcement of Earthbound (Mother 2) coming to Wii U's Virtual Console had you hoping that Nintendo might eventually continue the series, you might want to dial back the excitement. According to a tweet from Mother series creator Shigesato Itoi, making Mother 4 would be "impossible."
The tweet went out in response to a fan, who asked, "Mr. Itoi, please tell us a way to get you to make Mother 4."
無理。@whirluigi 糸井さんにMOTHER4を作らせる方法を教えて下さい。— 糸井 重里 (@itoi_shigesato) April 29, 2013
So, according to Itoi, that's a definite no. At least fans in the west have that Earthbound Virtual Console port to look forward to, and one can always dream of the day Nintendo decides to officially localize Mother 3.