Upgrade path D4s to D70 to ? (D600?)

D600 should work fine; my only concern would be putting your collection of lenses in front of 24mp body and your shooting preference.  I'd think the lenses will struggle and reveal all it's short comings (especially if you tend to pixel peep) if you tend to shoot with wide open apertures. If you shoot stopped down, I don't think it'll be an issue. That said, I also think 12mp FX body will work better with your current lenses if you do like to shoot wide open.