Mike,
The amount of smoke flavor is a highly personal taste issue. If you thought 4.5 oz of apple wood got a little bitter, next time cut it back maybe to about 3 oz. Or try pecan wood or alder wood. Point is to experiment. Too much of any wood will get bitter. Also, like Tom suggests, it could be something in your rub, like too much white sugar goes from carmelization to burnt (bitter) pretty fast. If you used white sugar, try brown sugar or turbinado instead.