You do know SMT3 is almost a decade ago right? As much as I am a fan of Persona, I say that a sequel to the main SMT is much more overdue than Persona.
Besides, Eternal Punishment and P3 had a 6 year gap, or 7 if you play the English versions. FEEL MY PAIN!
(For the record, I won't hold my breath for any more Persona rpgs for this generation's consoles, considering Atlus announced they're developing flagship SMT, Devil survivor, and Persona on the 3ds. They've confirmed 2 out of the 3, and since I don't see anyway for them to milk P4 by doing another spinoff, I'm pretty confident P5 will be in 3ds)
As for SMT4 itself, I personally don't care much about the main series, as having your battle party mainly composed of demons isn't my cup of tea (I like party members with human-like personalities).
What I do care about is that the flagship series is being released on a handheld. Which doesn't bode well for the lesser spinoffs getting a console release. But I guess that's to be expected, where it just costs too much to develop for consoles when RPGs on the whole are becoming more and more niche.