If a topic is hidden and a user (teacher) 'switches role to student', then they can still see the topic greyed out in the course index (not expected behaviour).
 

 
Clicking on the topic 'Database testing' the following error is displayed.

Which would be expected as the student user should not be able to view or access that content. Clicking on the database activity a similar error is displayed.
 
When doing the same as a site admin, the topic and content is sdisplayed in the course listing as above. But when accessing the content it is displayed without any errors. 
 
It appears that moodle 4 is not respecting the switched role but is still applying the permissions from the original user role.