4 ounces should not be too much hickory for ribs, but everyone likes different amounts of smoke. Especially since they were really only exposed to smoke for 3 hours. Most likely your wood was too dry, and maybe caught fire. This might give you an old ashtray flavor. What are you using for wood? Wood from big box stores like Weber or Western is usually bone dry. You might consider trying smokinlicious.com double filets. You also might want to experiment with cherry or maple, which are a little milder. Also, if there are any leftovers, try them the next day and see if they still have that flavor.