Fullscreen window across multiple displays

I'm trying to play video in fullscreen with mpv player on 4 equal displays combined as one. It works with the nouveau graphics driver, but the framerate is very poor. I want to make it work with the proprietary nvidia driver but the window extends to just one display. I'm using

xrandr --setmonitors 

to combine the four displays. Any ideas?

UPDATE:

I found that I need custom xorg.conf file in /etc/X11/xorg.conf.d/ It can be generated by nvidia settings and tweaked a bit. I changed: "Xinerama" to "1" and "BaseMosaic" to "on"

In my case this configuration works for 2 monitors. I will test it with 4 in the near future.

 nvidia-settings: X configuration file generated by nvidia-settings
# nvidia-settings:  version 390.144

Section "ServerLayout"
    Identifier     "Layout0"
    Screen      0  "Screen0" 0 0
    InputDevice    "Keyboard0" "CoreKeyboard"
    InputDevice    "Mouse0" "CorePointer"
    Option         "Xinerama" "1"
EndSection

Section "Files"
EndSection

Section "Module"
    Load           "dbe"
    Load           "extmod"
    Load           "type1"
    Load           "freetype"
    Load           "glx"
EndSection

Section "InputDevice"
    # generated from default
    Identifier     "Mouse0"
    Driver         "mouse"
    Option         "Protocol" "auto"
    Option         "Device" "/dev/psaux"
    Option         "Emulate3Buttons" "no"
    Option         "ZAxisMapping" "4 5"
EndSection

Section "InputDevice"
    # generated from default
    Identifier     "Keyboard0"
    Driver         "kbd"
EndSection

Section "Monitor1"
    # HorizSync source: edid, VertRefresh source: edid
    Identifier     "Monitor1"
    VendorName     "Unknown"
    ModelName      "Chi Mei Optoelectronics corp."
    HorizSync       68.0 - 68.0
    VertRefresh     59.9
    Option         "DPMS"
EndSection

Section "Monitor2"
    # HorizSync source: edid, VertRefresh source: edid
    Identifier     "Monitor2"
    VendorName     "Unknown"
    ModelName      "DELL P2421D"
    HorizSync       29.0 - 113.0
    VertRefresh     49.0 - 75.0
    Option         "DPMS"
EndSection


Section "Device"
    Identifier     "Device0"
    Driver         "nvidia"
    VendorName     "NVIDIA Corporation"
    BoardName      "GeForce GT 540M"
EndSection

Section "Screen"
    Identifier     "Screen0"
    Device         "Device0"
    Monitor        "Monitor0"
    DefaultDepth    24
    Option         "Stereo" "0"
    Option         "nvidiaXineramaInfoOrder" "DFP-1"
    Option         "metamodes" "LVDS-0: nvidia-auto-select +0+0, HDMI-0: 1920x1080 +1920+0"
    Option         "SLI" "Off"
    Option         "MultiGPU" "Off"
    Option         "BaseMosaic" "on"
    SubSection     "Display"
        Depth       24
    EndSubSection
EndSection