Seriously. Almost a year ago the same question was asked, and OOO responded with a very vague "Soon." We had no idea at the time what soon meant, and now, almost a year later, still have no idea what it means.
So when the hell is Tier 4/The Core and the floors beyond D29 coming out, exactly? We ARE getting this at some point, aren't we? T3/Shadow can't be the end-game forever.
You won't really be getting a better answer than "soon", to be honest, considering how secretive Three Rings is about updates. Specially something as major as this. I don't think The Core is really high in their priority list right now, since they have gone almost completely silent about it after the game was released, and remain silent to this day.
And I wouldn't be so sure about T4 or any new tiers at all, either. We have already gone far too long without those, they never really answer questions regarding it, and how Shadow Lairs work is kind of an indication they don't really care enough about new tiers.
@Shidara: http://i.imgur.com/jUO22.png
I think the reason that Three Rings is that The Core would be the "Endgame". After doing that, a lot of people would feel that they've done everything and stop playing, which would be bad for Three rings.
...At least, that's what I remember hearing.
The core can just as easily be a portal to other worlds and totally open up the SK universe to planets besides Cradle. The core is not by definition end-game simply because it is an unwritten page and OOO can write the story however they see fit at this point.
Sooner or later, it'll come. Have patience.