Ray, that is extraordinary! I've never seen this document before, and I'm amazed by it. I'm amazed because I spent many years as an electronics engineer and I'm very familiar with what the various microphones, etc, look like. I've had my 1520 stripped right down maybe four or five times now, and I can state with confidence that there are only two transducers - one at the top and one at the bottom.
Both are dual-purpose - they can both make sound and act as microphones. For what it's worth, when I was experimenting covering them up, I found it almost impossible to stop them picking up sound. As a result I reverted to an old trick - blowing. If you blow very gently (as silently as you can) at a microphone it will pick up plenty of "white noise", where the air stream interacts with the mic diaphragm directly. Nearby microphones, though, don't pick up anything because the air stream is not directed at them. I think if you repeat your experiments with gentle blows rather than quiet sounds, you will get more useful results.
I confess I cannot understand why that document talks about four microphones. However, it reads very much like it has been written by non-technical people, and in a very "young" tone. I suspect this came out of the marketing department rather than the technical department, to be honest.
Perhaps I might appeal to your common sense (instead of expecting you to just trust me). I think you will agree that there are many reasons not to fit four microphones in a phone. One is the pressure on space, another is the additional cost. There is simply no reason to have two microphones at the top and two at the bottom, when one at each end will work just the same (omnidirectional mics will pick up sound from both sides with ease). Furthermore, those mics are actually dual purpose and will work to generate sound, as well. Thus we have reduced it from four microphones (plus, presumably, two loudspeakers) to two dual-purpose transducers, one at each end. That is EXACTLY what any production engineer would do, and Nokia are pretty close to the king of production engineers.
Anyway, the ultimate test would be to take yours to bits and have a look! Referring to the video, you just get a glimpse of the lower transducer at 17m 18s; it stays in the black plastic piece that is taken off screen. At 9m 25s you can see the "earpiece" (the upper transducer) being removed.
Thanks, Ray, for pointing out that document. I had no idea it even existed.