I'm with jb on this one and he has you covered on everything. I'm not sure how you know about the 4by4 beams. Doors can be put into load bearing and non load bearing walls with no problem as long as the proper header is used otherwise you wouldn't be able to build houses without doors in them.
I'd just start cutting drywall (gently so you don't hit any electrical wiring which is most likely in the area 1 to 3 feet from the floor, if at all) in the place you want to put the door to see what you have. Then you will have to cut more to add your king, jack, and cripple studs and your header.