How to write formula topcount with adding 4 members in filter?

[abcuser]

Hi,
using Cognos v8.4.1 Report Studio accessing Cognos TM1 v9.5.2 cube I would like to write a formula to get top 3 customers for quantity measure, but I would like to make a filter to use only 4 specified products.

So far I have done:
topcount([LevelBuyer];3;[Quantity])  --> returns top 3 buyers for quantity
topcount([LevelBuyer];3;tuple([Quantity];[product_1])) --> returns top 3 buyers and make filter for product_1.

But what I really need is filter for 4 products: [product_1], [product_2], [product_3] and [product_4] and not just [product_1].

How to add filter for additional products into topcount function?

Thanks