If we look at the referential integrity of these two tables, what would happen if a new staff member was hired, we added a row for that staff to the staff table and gave the store$\_$id the value of $4$ as a foreign key value for that new staff member?

You could add the row successfully but your database would not be related properly.

You would not be able to add that store id value of $4$ because there is no store with a primary key of $4$ in the related table.

A $4$th store would automatically be created by the system.