According to Epson UK, you'll need to run your print application under Rosetta in 32-bit mode, as they've yet to release an updated driver for the Stylus Pro 4000 -
http://www.epson.co.uk/Printers-and-All-In-Ones/Large-Format/Epson-Stylus-Pro-40 00/Drivers-Support?target=article&extn=.html&articleId=3561
Not sure if the above works as I'm using using Aperture 3, which doesn't have the option to open in Rosetta. So, what I've done is to partition a drive, install a version of Leopard (10.5) and use that as my operating system when I need to print. Under Snow Leopard, print management options are disabled in Aperture 3.
According to Epson tech, they are working on a print driver for the 4000, and that it will be released in the "Spring", whatever that means.
For some more info on this problem:
http://www.luminous-landscape.com/tutorials/solving.shtml
http://www.luminous-landscape.com/essays/woes.shtml
Hope this helps...