If you go to Main Menu > DISPLAY > Custom Split in your NVR settings you should be able to select cameras you fill in those 4 remaining live view windows.
See page 423 of the Dahua NVR User Manual (section 5.16.3 Custom Layout) for more details on how to configure the live view layout.
Have you checked that your NVR is running the latest firmware? The Dahua website appears to list a V4.002.0000000.7.R.240717 firmware that looks like it was released a couple of months ago.
If you are not running this version, try updating to this version and then check the DISPLAY settings again (the above user manual came from the Dahua product page I've linked above so those are definitely the correct instructions for the model you have but you could be missing these settings because you are running an older firmware.