I've just started testing my channels on Roku 4 and found that some MP4 video streams are playing audio ok but the screen is green rather than showing the video. The particular videos I am testing with have worked on Roku 3 and Roku LT, both in HD and SD modes. I'm still investigating but the first thing I looked at was Content Meta-data.
I tracked this down to a bad interaction between roSlideShow and roVideoPlayer. In my application I put roVideoPlayer in the screen stack on top of roSlideShow. Up through Roku 3 hardware, there was no problem with this. On Roku 4 specifically, I get a corrupt video for roVideoPlayer unless I close the roSlideShow first.
I'll be working on a workaround but it would be nice to fix roSlideShow to behave better in a future firmware update.