I recently did get the US Edition of Season 4 as well and the DVD Quality for all the disc were perfectly fine for me. Now mind you it wasn't Blu Ray it was just normal DVD Editon but everything is fine. For your editon it might just be a bad disc in the end or it could be the Blu Ray Player. Sorry I couldn't answer your question exactly the way you wanted since my editon wasn't Blu Rays. However I just thought I could help a little since I recently got the Season 4 DVD as well.
An on a brief side note in was weird seeing The Sword in The Stone Part 2 on the one disc with the special features. While the rest of the Episodes were all combined together on the other discs.