Digital BW, The Print

Here is an excerpt from the epson site that makes some subtle 
distinction on the suitibility of these two printers for B & W 
printing...

4000
Professional Level Black & White Photographic Printing¹ 
*   Produces a truly consistent image with little color crossover or 
colorcasts 
*   Reduced metamerism on 7-color black & white prints 
*   Depending upon media, produces a black D-max up to 2.1 
*   Produces sellable quality neutral or toned black and white prints

7600
Professional Level Black and White Photographic Printing
*   Produces a truly consistent image with no color "crossover" or 
color casts. 
*   Able to produce neutral or toned black and white prints to 
satisfy demanding photographers. 
*   Depending upon the media used, you can produce a black D-max up 
to 2.0.

Draw your own conclusions.... I personally believe that these subtle 
distinctions in Epson's product literature indicates a difference, 
for some reason, in the B & W printing quality of these two printers.

Sincerely,

James Sims
http://www.jamessims.com