I am in the process of re-designing my website. I will be changing the URL structure of several pages and in some cases the pages are going to be deleted as they are no longer necessary or obsolete.

My question is this:

I do not want to effect any SEO by having a lot of 404 errors pop-up. For the pages that I am changing the URL I will set 301 redirects but how do I handle the pages that I am deleting outright and have no redirects? From my understanding if those pages start showing 404s it will hurt SEO. Is this correct? How do I handle deletion of pages?