1. Create a dataset to populate the fourth parameter
2. This dataset must include the dependent parameters (i.e. Region) as filter or where clauses
3. Create a new report parameter and select the newly created dataset in the "Available Values" section and select the key-value pair columns from the same section.
That's it, now you should able to see the values in 4th parameter only after selecting a value in the region filter.
--Ramesh