Stress Marks by all four corner screws

Hey iFixit! Long time reader first time poster. I just finished working on a screen replacement for a iphone 4 when i'm noticing stress marks around all four corners. These stress marks are only found by pushing in the exact corners right above where the four corner screws are located and nowhere else. Any ideas on how to stop this from happening? I tried taking apart the phone and putting it back together but it didn't really help. If anyone wants images I can upload them later.