Well, without knowing the exact security posture that you intend to incorporate into your setup, it's tough to say much. However here is some information that will help you do the same. The simplest to secure your network is to configure acess lists. Many of the common attacks use address spoofing, something that can be easily prevented using access lists. For example, a packet coming in form the outside interface carrying a source IP which is being used on the inside, is in all likelyhood a malicious packet. You could use access lists to deny such packets. The range of attacks which you can insulate against using access lists goes on. For more information see Configuring Network Security (Using ACL's) at http://www.cisco.com/en/US/products/hw/switches/ps663/products_configuration_guide_chapter09186a00800ddcfb.html
The document 'Configuring Network Security' at http://www.cisco.com/en/US/products/hw/switches/ps663/products_configuration_guide_chapter09186a00800f0e24.html has security information that is unique to the Catalyst 4006.
Also see, Security Overview (for 12.1) at http://www.cisco.com/en/US/products/sw/iosswrel/ps1831/products_configuration_guide_chapter09186a008008824a.html.
