Good Morning, SoFaKingCity:
I am going to have to agree with Inspector D and Square Eye. Put another joist in between each 4' space, suspend it with 5 2 X 4s on each one to stabilize it and use gypsum wallboard for the ceiling. If your garage door would interfere with that ceiling; you may want to consider raising a section of ceiling to clear the door.
It will take some extra work but it is not hard work, just time consuming.