Lee,
The plenum is the inlet area feeding the (4 port) manifold, correct? The inlet at each manifold is "blah" inches long. Your plenum is a piece of tubing cut by a chord to fit your manifold plate, correct?
If so, then you have a problem. Your "tube" is a cylinder. It needs to be some other shape. There are many it could be. The "issue" is the Ideal Gas Law (PV = nRT where "P" is (absolute) pressure, "V" is volume, "n" is the number of molecules, "R" is the "gas constant," and "T" is the (absolute) temperature). At "steady state" the "nRT" side of the equation is a constant. Thus, what you are trying to do is to maintain a constant "PV" across each of the manifold inlets. Correct?
So, the "PV1" at the first inlet needs to be 4* the "PV4" at the fourth inlet. [You also need to maintain a "smooth transition" in between each inlet to avoid "flow losses." Otherwise you lose the "assumption" that "P" is constant along the length.] Etc.
The "issue" is going to be figuring out the shape that fits your application. I normally do this type of design for hydraulic circuits that are quite a bit simpler than pneumatic circuits.