Much ado is made of optimizing URLs for SEO. However, the real importance of URL shaping is preserving bookmark and external link traffic. Keyword-stuffing URLs is an old, cheap trick, and search engines give it very little weight because it's so easy to abuse.
And it makes stupid URLs that are hard to type. Even more laughably, breaking external links breaks PageRank/link relevance. These days every search engine also makes a browser, so with a little snooping, they can also use user bookmarks to rank. So preserve that organic traffic, it is more than organic. Some people say "never 404 any page"; I heartily agree and this is why.
There are no 404s in my log, except for human-fatfingered URLs, and that's usually me lol.
Of course, reality is that you do change web platforms from time to time, and you get locked out of using your traditional URL. That's where the HTTP response "301 Permanent Redirect" comes in, because Google and Bing certainly understand a 301, and PageRank and other factors will follow the 301. Upshot: 301 is essential.
Some lousy websites redirect all 404s to the homepage. Seriously, how often do you then stay on that website? You don't, you leave. So does Google, they know that you have destroyed the specificity that their searcher wants, so the search engine discards the page-level rank factors. The site had them, the site wasted them, and they get to start over.
Anyway, what you propose is exactly the right thing: redirect content to its specific mate page that has the same content, or at least content that is equally specific.
Your hope is that the "pretty URL" will provide better SEO. I don't put a lot of stock in that, as said. However, I certainly do put stock in the search engine doing a better job indexing and ranking "static" pages like
site.com/dir/subdir/page.html than dynamic pages like
site.com/engine.cgi?param=value¶m=value. The latter is often redundant to static pages elsewhere, search results of an internal search engine, or a black hole (infinite, recursive links). Because of the diminished value, and risk of black hole, crawlers are skittish about indexing those deeply.