Joining The Army after Discharge (general under honorable)RE code:4?

I got a Article 15 last year for drug use. They gave me a Article 15 for that. Loss of rank, 45 days extra Duty, loss of pay. pretty much they maxed me. i was suppose to got to counseling for that incident and never did due to field exercises and other things that were going on. I had Appointments but missed them due to that my 1SGT at the time said don't worry about them because he was going to reschedule them but never did. After the extra duty and my article 15 they started chapter paperwork on me. to summon it all up i got chaptered out of the Army july 22, 2010 with a general under honorable conditions with a re-entry code for 4a. I had no hearing or anything like that i was doing some research and one article said that it would make my chapter invaild if i got no hearing. So what would be the next step or steps i would have to do to change my RE code or join back in the Army if someone could help me i would greatly appreciate it