I don't know that you can do anything. The thing about cracks/scratches is that it is very hard to show that it wasn't your fault but rather a manufacturing defect. The fact that its been 4 weeks when you just noticed it has me thinking that the best they will do is make you pay to fix it. That won't be a warranty type of thing.
With that being said, I know how you feel spending that much money and having something like that. The thing is, it's on the back of the computer and you can't even see it while using it.
I had a bigger chip between my speaker grill and keyboard that you can easily see while using it. Luckily I saw it and took it back after a few days of receiving it and having decided that I couldn't look past it. Since I was in the 14 days, and the round pin hole like chips in mine couldn't have been made by me unless I took an actual pin and dug into it which I obviously wouldn't have done. With that being said, they were really cool and are changing my case for free and no problems.
I think the hardest part in your case is that it has been a month and those chips are in locations that are likely to be damaged by the user.