How to connect 4 monitor to 1 Rasberry PI 4 ?
Raspberry Pi 4 video supports dual 4k HDMI (micro hdmi sockets). Also supported is composite output but it's not clear if this can be used simulataneously as the HDMI ports. It certainly wouldn't be desirable to do this due to processing overhead and visual quality differences between the HDMI and composite outputs.
Whilst it may be possible to additionaly use USB to HDMI (or VGA) outputs it's unlikely that there would be enough processing resource to make it practical.
2 monitors is the practical limit.