Hi Jeff.
We utilize the same motor but with a different gear ratio to be able to handle the extra weight of the 12LB drum. It works great and we have had no problems. The motor does, however, spin at 25RPM and has no issues obtaining even roasts at this speed. If one attempted to roast with the smaller drum motor then yes it would eventually burn out. You can roast as little as 3/4 LB on the 12LB unit so definitely, 4LB will be no problem. Let me know if you have any other questions!
-Shane