Nintendo just recently registered a domain for Super Mario 4. If everyone recalls Nintendo did make an announcement of a 2D Super Mario Game in the works for the 3DS during their latest financial results briefing back on 01/26/2012 and more than likely we will see this game at this years E3 in June.
Here is what Iwata said back in January:
“Although there is no logo here, we are planning to release a totally new side-scrolling action Super Mario in 2D as a key title for the Nintendo 3DS in the next fiscal year.”