>When the D300 came out, there was no D700, and >the full-frame D3 cost four times as much.
No, not 4 times. The D300 launched at about $1,800 and the D3 at about $5,000. That's "only" 2.777 times as much
If the same pricing relationship were applied to the D4 and a potential D400 (that might be a big "if"...), a D400 would be around $2,150 compared with the D4 launch price of $6,000.
I don't see why a D400 has to be in a completely different price segment from the D600. They would be aimed at different types of customer - someone who wants a "pro" DX body isn't in the market for a "semi-pro" FX body, and vice-versa.