Don't pre-heat the plank.
Cooking times will vary based on how hot the fire is and how far from the fire the plank is, etc. Also, the thickness of 4 pieces of filet that total one pound will not be very different from a single piece of filet that weighs three pounds. (the 3 Lb piece will be longer but not thicker). I would expect the individual pieces to cook a little faster than a single piece because the heat can get to more edges on the single pieces. But I'm talking 10% less not half the time.
As always, the internal temperature is the final determinant, not elapsed time.
"If you want to make an apple pie from scratch, you must first create the universe." -Carl Sagan