The only way you could do this would be to have the 4th parameter get its available values provided from a dataset, which has the 3rd parameter (and any others it may need) provided to it so that it only returns valid values.

When you load the report doing this, the 1st, 2nd and 3rd parameters will all be available, but the 4th will be grayed out. Once you select the compulsory parameters, the 4th will be available for selection as well.

For example if you had a table as below:

Insert into [SSRSExample] (Name, DeptID)

Values (N'Steve', 1),

(N'John', 1),

(N'Dave', 2),

(N'Gina', 2),

(N'Paula', 2),

(N'Daniel', 3),

(N'Bobby', 4)

Your 3rd parameter supplies a distinct list of DeptIDs, so you would have a dataset that has the following query to provide available values for Parameter 4:

Select Name

from SSRSExample

where DeptID = @DeptID

Providing 2 into the dataset would cause parameter 4 to only provide the values Dave, Gina and Paula in a drop down box.

Does this help/make sense?