How big is the motor?
3 or 4 inches sounds like a lot for that technique. One or two probably would be OK. I did that with a 1 1/2 inch block and a 6 HP motor for years on a sailboat.
Another question is where are the bolts/clamps that hold the motor onto the boat. I would not rely on the "2x4" if it is where the clamps hit.
Here is something to consider. How about making a sandwich of PT wood and a few pieces of 1/8" to 3/16" Aluminum plate. Bed everything down in 3M-5200. Even one piece on the outside would really help.