I'm just arrived at a sector on a 4 year billet. I enlisted for 4 years so I have a little less than 3 left. Since its a 4 year billet do I have to stay there all 4 years or am I done once my enlistment is up?

Your done if you get out. AO get a tad upset when you pull the transfer me, or I will get out. My AO will tell you, have fun.

Yes. You will stay there until you get out, or until you are tour complete. If you re-enlist, you will have to complete your tour there unless you advance or the detailer needs you for a critical fill billet.