Hello,
I think that you have different way to proceed :
1. If your client's customer want to buy 4 same products, you'll just have to :
- go to " Hikashop -> products -> 'Your product' "
- and add a new price for a minimum quantity of 4.
2. Else if your client's customer want to buy 4 different products, you'll have to :
- go to " Hikashop -> system -> discount "
- create a new coupon
- set the "Percentage" to 25, the "Minimum number of products" to 4
- set all of the "shirts" in "Product", or/and simply set the "shirts's category" in "Category"
- set "Auto load in cart if possible" to YES
- and finaly set "Coupon percentage applies to product only" to YES
Hope this will help you.