You could try the method where the board is supported with it's longest edge on the floor and a cable, up to 8+4=12' long supported at the top corners with a motor and pulley. The pen is suspended in between, so the cable makes a V-shape. By adjusting the length of the cable by spinning each of the motors you can get the pen to move to any position within the board. It's simple trigonometry to work out the relationship between pen position and motors. Then you enter those formula's into mach3 and it will run it as normal...
A standard belt driver plotter will probably be better, but since you're going to have to buy steppers and drivers for either you could try this initially as you only need a piece of string and a pen...

Edit: Wrote this post 2 days ago but forgot to press send!