Reporting Services :: SSRS 2012 Setup Parameter Value

In an existing ssrs 2012 report, I want to basically add the following logic to a new dataset that will be used by a new parameter called 'Inventory'.

SELECT  DISTINCT Inv_number as value,  Inv_name as label
FROM s.dbo.fnUser( @endYear, 0, 0, 0)
WHERE (@report is not null)
order by  label,value

In this ssrs report, there are 15 reports that can automatically be generated. I basically want this new parameter called 'Inventory' to only be active when the 4th report is selected to be generated. Thus in the query I listed above, how can I change the query to only be active when the 4th report is selected?

I tried to change the where clause to (@report(4) is not null), but that did not work.