i dont know about site 4 but with the original you can put a milti kit in it..
theres a guy on fleabay that sells roms, security chip, and hd that will convert a area 51 board into a area 51/ maximum force. 2 games in one, on screen selectable, very, very, very well done kit. Whoever out in the time to make the "duo" kit did a fantastic job!
i love it. i got my kit about 6 mos ago now and im VERY glad i did it.
so check fleabay and give that dude a email and see if his kit works on the site 4 board as well...