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Hello and thank you for your question.
I can understand how concerned you are about your son, and also how frustrated.
It does sound like he definitely needs professional help to overcome his addiction to marijuana, and yes smoking it 3-4 times per day, is a major addiction.
Talking to him is not working as you know all too well, and not only does he need therapy with an addictions specialist, but most importantly he needs to want to stop and have the will to do such.
I do think that giving him a consequence to his behavior (moving out) is a very good idea. You don't want to enable this behavior. He needs to see a therapist who also specializes in addictions. Here is a very inclusive list of therapists who are addiction specialists, all verified. You can search by zip code (for his college area) and also by your area. http://therapists.psychologytoday.com/rms/
Finally, you can get support by joining a group such as Al-Anon that is for family members of those suffering from addictions and/or finding support and resources online. Here is a really good resource for family members (like yourself) dealing with marijuana addicted kids. http://www.mdjunction.com/marijuana-addiction
Also, please remember to click on the green ACCEPT button for this answer. Feel free to continue the discussion with me, even after clicking ACCEPT. Thank you.