Assert the page in the browser is (or is not) a "page not found" page: the site answers its URL with the 404 status. A page that is not published answers 404 to a visitor.
Examples
Then I should be on the page not found page
Then I should not be on the page not found page
And we should be on the 404 page
Then should be on the 404 page
But we should not be on the page not found page
Pattern
Then(/^(I |we )*should( not)* be on the (?:404|page not found) page$/, …)
Part of @webship-js/django-cms-steps (github.com/webship/django-cms-steps), a step pack for Webship-js 2.0.x.