What you want to do won't work.
As far as I know, it isn't possible to use an Intel CPU's graphics and a discrete graphics card at the same time. The CPU's graphics must be disabled to use a discrete graphics card, and a discrete graphics card must be disabled to use the CPU's graphics. You need to install two low-power graphics cards to run 4 monitors, or you need to install a more powerful graphics card that can run 4 monitors on its own.
Some laptops have "GPU switching" so that it is possible to easily switch between the discrete graphics and CPU graphics (to conserve battery power) without going into the BIOS, but I have not heard of any desktops with that feature.