So it must be an issue with the current DAG size (almost 4GB) and your OS must be using some of the GPU's memory.

If that's the case, try to set the onboard GPU as the main display in your BIOS?
Perhaps, this might reduce/remove the memory resource usage of the main GPU.

Of course, you should connect the DP/HDMI/DVI/VGA cable to the onboard video port.
If you end up with issues, you can always reset the BIOS.