Hello,
Here is some very useful info on our NVR-SB64...
For starters I'm sure you have realized that this NVR supports two HDMI outputs. It is very important you know HDMI #1 is NOT the main display, HDMI #2 is in fact the main display, while HDMI #1 is the secondary display (Video Matrix). That being said, only the main display (HDMI #2) can support a 4k output, the secondary display (HDMI #1) only supports up to 1080p output. Please see the images below.
In Addition, you should also know that you can only watch up to 32 cameras at a time on both main and secondary displays. However, on the secondary display you can enable a tour to change between two sets of 32 cameras (1-32 & 33-64) if necessary. You can also do cameras 1-32 on the Main display (HDMI #2) and cameras 33-64 on the secondary display (HDMI #1) if that suits your needs better. There are also many many different sets of tours you can enable under the secondary display. Please see images attached.
Hope this is helpful for you. If you have any questions/comments we would be happy to help. Thank You!