Easiest to enter it to each product that it applies to and/or in the category description of they are all in the same category.
I also would have to change each glass item to multiple the price by four.
The forcing of set of four can be approached in different ways, you can leave it as you have "Box of 4" but if you think that from a marketing aspect that displaying a lower single glass price is the better option then when you set this up or now go back and edit the products and find and set the min to 4 or 6 and the units to the same:
Code:
Product Qty Minimum: 4
Product Qty Maximum:
Product Qty Units: 4