Standard Industrial Classifications (SIC)
This page, run by the U.S. Department of Labor's Occupational Safety and Health Administration (OSHA), offers two ways to find 4-digit SIC codes: by searching on keywords for a specific code and by browsing the actual classifications. Note that SIC codes have been superseded by the North American Industry Classification System (NAICS) on January 1, 1997 (information on it can be found at the Census web site).
http://www.osha.gov/pls/imis/sicsearch.html