Both options are valid and more of a principal discussion. As the page hasn't ever existed, a 301 redirect (moved permanently) may not be the right option, but from a user perspective, the /about may (or may not) be redirected to most important item under that group.
a 404.0 (page not found) may the better option from a principal perspective, as, the page doesn't exist. Users may think that it's a badly build site, as a valid looking url results into a result that wasn't expected. (Is the site broken, do I miss information?). Especially because /about is part of the URL, and people tend to remove parts of the url to navigate through the site (well, I do ;)).
I would choose the 301 redirect in this case, for those "group" url's, as they are part of the url's of the subpages. I would use 404 for pages that really don't exist, like /About1, /bla, /huppeldepup
as the question got changed as well ;)
for this example, using the hero slider content, I would choose for a 404, as it are url's that people shouldn't navigate to.