This approach to the rule won't work unfortunately as the "Discount Qty Step (Buy X) " only applies to multiple quantities of the same product. Hence why it works if you have 4 of the same item but not if you have 2 and 2.
You could create a rule which has a condition on it instead which looks for cart quantity of 4. This isn't going to automatically work out the multiples though. Whats the range of order quantities of your stores? If it's not too high, you could manually create the discount levels with multiple rules, each with their own quantity condition, i.e. one for 4 items in the cart, one for 8, one for 12, one for 16.
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