This is because a 4K TV will force the BDS into 4K resolution (upsampling), because this gives the best picture quality, being the highest resolution possible. However, there is no such thing as a 3D 4K movie. All 3D discs are full HD (1080p), but cannot be reproduced in 4K, because 4K resolution cannot be combined with 3D. The solution is to enter the setup menu of the BDS, selecting normal full HD resolution in the picture settings menu. Now 3D movies will be shown correctly on the 4K TV.