Best Place to 404 a Non-Matching Custom Query Var

I’ve created a ‘tabbed’ layout on custom taxonomy archives on my website and successfully rewritten my permalinks so that visitors to see some conditional content as a child of the term. There’s currently about four tabs in action.

However, it’s just come to my attention that a request to a non-existing tab resolves but is empty because it doesn’t meet the get_query_var() requirements. My question is, what is the best method to 404 any requests to a tab which I haven’t explicitly declared any content for, like Should it happen at the point of redirect rule creation or at the point of content output?