To do this you can add tier prices for the product.
Go to the product page in the Magento admin area and navigate to the 'Price' tab. There you will have an attribute called 'Tier Price' where you can configure the price per ordered quantity.
Edit: As per request a solution with shopping cart price rules:
You can do this by making 2 separate rules.
Rule number 1:
Rule number 2:
Now make sure that rule 2 is executed before rule number 1 by giving it a lower priority and you're done :-) Rule number 1 won't be executed because rule number 2 has 'stop further rules processing' enabled.
You can of course change the conditions to match an attribute like partial SKU or attribute_set instead of a category. This is up to you.