I have a Roku Ultra 4k connected to a Sony Bravia XBR-65X850D 4k TV, and every once in a while, the display will double up, and I have to reboot the Roku to clear it up. This seems to be a known issue, because I found the same issue here
Those are the old user forums that are in archive/read-only mode now (you cant post there, you have to post here, in the new user forums)
Other than trying a different cable or connecting directly to the Roku (if you arent) and assuming you have the latest Roku firmware (9.4.0-4200) for your 4640 and for your Sony TV (v6.827), then you'd want to contact both:
Can you please provide us the following information: -Roku device model -serial number -device ID -software OS/version (these can all be found in Settings > System > About) -does this issue occur on a specific channel? if so, what channel and what version/build is the channel (this can be found be selecting the channel on the Home screen and pressing the * button) -tracker ID when this issue occurs (when you see this issue occur, press the Home button 5 times, followed by the Back button 5 times)
With more information we will be able to look into the issue further.