US army contract and Special Forces.?

I leave for the army in Oct, I signed my contract and everything. I did 11B and airborne Contract, My Serving time was 6 years Active Duty. My goal is to become a green beret after 4 years. My dad is a Colonel in the Army, so he said he can get me stationed at Fort Bragg, 82nd airborne Division. I wanted to do infantry first because I wanted to get some experience before I tried to become a green beret. I was wondering after 4 years, can I change my MOS to the 18 series/ or Special Forces?