I was up in the attic today. When the insulation is pulled away you can see the support beam from the top of the wall. The plaster ceilings but up to it. The joists in the attic overlap above the 2 x 4 beam. In the area where I plan to put the pocket door they are 2 x 6's. Then off to the side there are 2 x 6's attached to 2 x 4's. Not sure if this last info is relevant or not.
I'm not sure I can understand the description, however, the general rule is(majority of times) that the a center wall of a home is the main load bearing wall.In order to make adjustments to create wider openings, the ceilings on both sides of that wall, should be braced/shored, in order to support the two ceilings, which are generally supported, by that center wall.
Dependent on the number of floors above that wall, there is greater load bearing down.
As stated, since you feel, and it appears, that the wall us a load bearing one, I strongly advise that the re-framing portion of the project be performed by a professional - framing carpenter, remodeling contractor, or GC.
The contractor would install the pocket door frame, at the same time the wall is framed in with the new header.
I list those kinds of contractors for a reason.
That is because a "carpenter", is not necessarily experienced as a "framing carpenter".
A "Contractor", may be more experienced with drywall, painting, & minor trim carpentry, but know virtually nothing about structural framing.
So be careful who you bring in, to do the work, make sure that they understand, know, and possess the experience to do such a job, and are not just "yes-ing" & "uh-huh-ing"....you.