If your on Mac OS X, put ArraySync into Spanned mode, enable Offset, and then Set the offset to the dimension of the other displays on each computer. So if you're doing a 2x2 grid, you'd configure ArraySync on the upper left quadrant to have a right offset and bottom offset equal to the width and height of the remaining column and row of displays.
Unfortunately this isn't available for WIndows users.
I am now telling the computer exactly what it can do.
@Turboladdade Thank you for this information.
So I have to do this on every client computer in order to make the video wall. Do I have to do anything with the server computer?