Heh, I think Tactics is more "part of the core series" than F3, or even FNV (OK maybe on the same level as that). They can call it whatever they want, it's still going to be too spun-off from the roots to call it a proper sequel . More so in case of F4, at least it seems so from all the PR materials we've seen so far. Obviously it's not about just the name.
It's hard to find something to compare it to... kind of a unique situation I guess. It sort of reminds me of the Windows Mobile > Windows Phone process, except they at least changed the name (although that was mainly to tell WP and WinRT apart).
Only until you got to Vegas. Or maybe even after Nipton.
...
Oh well! I'll still going to call it a spin-off as it shows all signs of being a spin-off. And enough of the OT for me.
Brotherhood of Steel for PS2 is the best Fallout game. *runs*