Ernst,
... surprisingly the 4x00 kept that gravitation feeding ...
It's probably just cheaper -- no pump and simpler carts. The 4000 does appear to suffer some backflow. I suspect that was the source of lots of claims of clogs with the printer when it came out. The one I use never clogs, but if left for a few weeks without being turned on, I might have air in the head and line that gives a bad nozzle check.
Paul