Hi Chris
With over 4000 pages on your site I'm assuming you are using some kind of CMS? Either way I would recommend the use of the rel=canonical tag to counter this potential duplicate content issue.
It helps by a. proving to the search engine's you are already aware of this issue and b. clarify which of the duplicate pages they should prioritise and pass DA to.
SEOmoz have published a great post on the various options on how to implement the code which can be found here: http://www.seomoz.org/learn-seo/duplicate-content.
The other option would be to use 301's. In my experience implementing 301's on a window's server (assuming this is the case as your url extension is .asp?) can be tricky. The management of hundred's of 301's can also become very cumbersome and complicated often resulting in infinite loop errors.
Hope that helps.
All the best
Alex
