you 'need' to connect a 4th monitor? I recommend getting graphics card(s) to support your endeaver, but up to you.
Why don't you get a dvi to vga or maybe its a vga to dvi cord and connect the dvi monitor to the graphics card vga port?
Does it have to go into the HDMI port? HDMI carries sound as well as the video signal, dvi only carries video, and vga carries a lesser quality video