Unfortunately there is no ability to change visibility of a parameter dynamically. However, you can populate the choices in the Parameter selection dynamically via "Cascading Parameters".

You could do something like this when user chooses Report 4 or 5 from @Reports then the @sort populates with the sorting choices else it only has something like "Not Applicable". If your interested in this method I can show you some quick easy ways to do it if you post the code for those Params.

If you want to go complicated you could always create a button for each report that drills through to its own sub report and parameters. That's a lot of work though.