Shin Megami Tensei 4 was recently teased by an ad in Japanese magazine Famitsu and now on the interwebs. The Atlus published title notes on the teaser site the game is for 3DS and translates to "Shin Megami Tensei 4 - it begins."
This would be the first installment of a non-spinoff Shin Megami Tensei game since Shin Megami Tensei 3: Nocturne for PS2 back in 2004. Unless one considers Shin Megami Tensei: Strange Journey from 2010 for DS part of the main series.
The latest moves by the spin-off series' had Shin Megami Tensei: Devil Survivor 2 for DS launching this past February and Shin Megami Tensei: Persona 4 Golden on Vita planned for this autumn. And just because we didn't write it enough times in this post: Shin Megami Tensei, Shin Megami Tensei, Shin Megami Tensei.