You can run multiple GPUs to run more than 4 screens. This is pretty standard stuff. This isn’t SLI. This is just adding more display outputs. I do it in my setup (8 monitors). I have a 3070Ti as my main GPU, and an old Quadro P620 I use just to drive my cockpit’s touch screens. Works like a charm.
That said, it doesn’t work like you think it does.
This is incorrect. Only 1 card is going to be used by MSFS for its acceleration needs. The others are just passive display cards. The sim will not render across multiple GPUs or combine VRAM. All the heavy lifting of rendering is done the primary GPU. It’s what does all the compute and provides the VRAM for MSFS. None of the other cards are being used at all except as passive outputs. The main GPU renders everything and passes the rendered frames off to the other GPUs to display on their respective displays.
You’re basically wasting money and power driving 4 GPUs to run 4 monitors.