Not the person you're replying to, but my problem with the 4000 (and with "wave" and "bowl-shaped" keyboards in general) is that it is too hard for me to memorize the position and orientation of all the keys to the accuracy required for me to avoid RSI.
In contrast, in the old Natural keyboard, all the keys for the left hand inhabited the same plane, and all the keys for the right hand inhabited a second plane -- excepting the arrow keys and the keys above it, which inhabited a third plane. (And when pressed a key would always travel perpendicular to this plane.)
Consequently, once I determined where one key is, it was easier to know where its neighbors are because they are all in the same plane. Ergo I need to remember only 2 quantities, namely how far to the left or the right it is and how many rows forward or back, as opposed to 3 quantities -- or 5 quantities if the keys point in different directions like they do on the Kinesis Advantage and IIRC the 4000.
(And within each plane, the layout of the keys followed very closely to standard layout introduced by IBM with the PS/2, which also helped me keep things memorized.)
The locations of the keys on a Kinesis Advantage are even more impossible for me to memorize to the accuracy I require to avoid RSI!
Now might be a good time to describe my cynical explanation for the many reports that switching from an ordinary keyboard to an "wave" or "bowl-shaped" keyboard helped with recovery from RSI: the massive change in the layout forces the coder to play closer attention to where the keys are, which keeps stress on the coder's hands low. If my cynical explanation is correct, then coders who develop RSI while using a "wave" keyboard would improve if they switch to a standard keyboard (because again the large change in keyboard format would force the coder to play closer attention to the locations of the keys).
But one does not need to switch keyboard formats to pay closer attention to the precise location (and orientation -- the direction the key will move when pressed -- if you are unlucky enough to use a keyboard in which the orientation of each key is different) of each key.
How do you avoid stubbing a toe when you walk around your apartment without shoes? Well, I devote a tiny fraction of my attention to where my feet and all the obstacles are. And I do the same to avoid RSI while typing.