Vmix can normally output to 2 fullscreens. If you want more and your pc can handle this,you can go to settings and check Show Advanced Settings. Then go to options and check "Enable Multiple Instances". This will allow you to open another one vmix window. So in the first instance you can select in fullscreen 1&2 videos to go to screen 1&2 and in the 2nd instance of vmix you will select in fullscreen 1&2 videos to go to screen 3&4. Theoritically it will work but you need to have a 5th screen in which you will have vmix interface in order to have the ability to work your production
Or another solution is to use obs for your screens and vmix normally for your production