The reinforcement wood plate is provided for extra support for the wing mounting screw heads. As such, it needs to go on the bottom of the wing where the screws enter. The steps are:

1. position the plate on the wing and mark the perimeter
2. cut and remove the covering about 1/8" inside the marks and remove the covering, exposng bare wood
3. epoxy the plate to the bare wood. 5 min epoxy is OK here.

You noticed the wing dihedral doesn't allow the plate to lie flat. The plate should have a partial cut at the center that will allow the plate to be bent, or more accurately, cracked at that cut to let it lay flat on the wing.

After the plate is installed, carefully locate the measurements given in the manual for drilling the holes for the screws.

You say the wing attachment screws are 4-40? These kits use metric hardware. Recheck the instructions. Maybe they are 4mm by 40mm long?