It looks like the integrated graphics make use of system memory which makes sense since it does not have it's own VRAM. Now, driving 2 x 1080p monitors and the SP4's 2736x1824 could add up to a ton of memory used!
I imagine it doesn't take the full amount consistently for all monitors. you would have to be running like 4K videos on all three monitors at the same time perhaps. But just having a monitor sitting with an email open, another on a web page, and a third on a video isn't going overload the setup in discussion here by no means.