"Please be aware that Kodak has discontinued all support for the older models like the DCS 4xx cameras. Having said that, we would suggest looking for stand-alone graphic file converter software."
There are a few programs that can read the old DCS files, but they're often expensive and/or difficult to use (you need to download and install special support files, etc.)
So.. I spent a few hours and wrote the following piece of software (sorry, Windows only). Nothing fancy, but it works:

DOWNLOAD: NikonWeb.com DCS File Converter (2.1 MB)
Readme.txt:
NikonWeb.com DCS File Converter
ver 0.2 - October 2008
Free software to convert proprietary Kodak DCS TIFF and DCR files to ordinary TIFF, PSD or JPEG files.
The program is very simple to use:
1) Select the files you want to convert (select multiple files by pressing the CTRL key) or click on the Select All button to select all files in a folder.
2) Select the output format (TIFF, PSD or JPEG) in the Settings tab and specify a file name prefix
3) Click on the Convert Selected Files button.
The converted files will be saved in the same folder as the original files, but with the specified file name prefix and correct file extension (.jpg for JPEG files, and so on).
Supported camera models
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DCS 3xx, DCS 4xx, DCS 5xx, DCS 6xx, EOS Series, and NC2000 cameras
(Using the following Kodak support files: pdc_sdk.dll, ekfpixjpeg140.dll, profire.dll)
DCS 7xx, DCS Pro Back, 14N, and SCS 2000 cameras
(Using the following Kodak support files: DCSProBack.dll, DCSPro3SLR.dll, DCSPro4SLR.dll)
DISCLAIMER
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This is unsupported, AS-IS software. Use at your own risk.
This software is written by Jarle Aasland, and uses Kodak's own DCS support files and PolyImagePro by Larry Reeve. You may freely distribute and share this program, but you cannot charge anything for doing so.