Shigesato Itoi, who's best known as a copywriter and personality in Japan, already said that there will be no Mother 4. Making Mother 3 was no easy task. That doesn't mean there still isn't hope - however faint that might be.
In an upcoming issue of Japanese magazine "Brutus", there's a four page feature on Shigesato Itoi. The mag followed him around for two months in an attempt to pin Itoi, who's done everything from design shirts to voice the father in My neighbour Totoro, down.
Even though there are no plans to make the game and even though Itoi already said there will be no Mother 4, Itoi and Nintendo president Satoru Iwata discuss Mother 4 in the magazine. While the interview hasn't been published, don't look at this as any sort of confirmation. From the sound of it, it's just two guys talking about a game that doesn't exist.
One of those guys just happens to be Shigesato Itoi. The other is the president of Nintendo.
Mother was released on the Famicom back in 1989. It's sequel, Mother 2, was localised into English as "Earthbound". Mother 3 never received a Western release.