They're largely the same...
The PN4 and PN4A are pretty much identical on the inside. Brush rolls, motors, belts, everything is cross-compatible between the two models. The only differences are the 'motor humps', paint jobs, and body screws.
The PN4 has a motor hump that runs nearly the entire width of the unit, similar to its predecessor, the PN2, while the PN4A's motor hump runs just over halfway across. The PN4 came either in polished aluminum or painted chocolate brown to match the Olympia One. The PN4A came painted the aforementioned chocolate brown or in graphite gray with gray plastics, to match the Silverado.
The screws that hold the PN4 in place are slightly shorter than those for the PN4A. I decided one time to swap a polished PN4 cover onto a gray PN4A, and while it fit perfectly, I had to get new, shorter screws for it because the PN4 cover's screw holes were about an eighth of an inch shallower.
That same power nozzle came to me with a frozen motor, and I ended up replacing it with one for a later Omniflow power nozzle. I had to swap the back half of the case from the bad motor onto it to make it fit the PN4A. I was absolutely thrilled when I turned that thing on and it actually worked! It was my first, and thus far only, experience with motor work on a vacuum cleaner.
