The former A350 would have had a brand new cockpit section, new composites wings and systems.
The 330-400 would be a a330/340 derivative using existing A340 fuselages and mainlanding gears, center fuel tank and have new engine pylons / engines. And the latest systems / cockpit improvements of the A340-600, A332F and MRTT / KC-30. (info screens, bigger displays, hud)
Source:
http://www.!!!!!!!!!!!!!!/photo/Virgin-Atlantic-Airways/Airbus-A340-642/0866439/M/ , by Sam Chui
By combining existing family technology & a new engine a very competitive, cheap & Low Risk product with a short time to market could be realized IMO fitting in a market niche that proved significant lately with the A330-300 selling better then ever before.