Should I redirect old pages

[Roots7]

I have taken over SEO on a site. The old  people built thousands of pages with duplicate content. I am converting site to wordpress and was wondering if I should take the time to 301 redirect all 10,000 or so pages with duplicate content. The 10,000 pages all have links back to different  to different pages as well as to the homepage. Should I just let them go to a 404 page not found.

[john4math]

If some of those pages have some good external links pointing at them, I'd take the time to redirect those.  If they don't have much in the way of links, then I probably wouldn't bother.  Also, do you need to link them to specific new pages?  If they're mostly similar, and are generated in some consistent way, you could mass redirect them to one or more of your newer pages?