Regarding the extra "window" on BM4500;
the infrared-emitters can be seen by holding your camera/phone in front of the BM4500
press STATUS on f.e. Beolink 5000 and you'll see 4 infrared-emitters light up.
If i remember correctly, there are different versions of the BM3500 (perhaps the earlier models?)
that do have the same hardware as the BM4500,
but have different software, 2-way hardware installed, but disabled.
When i rebuilt a BM4500 into a BM3500, it was a version without the 2-way hardware installed.
The BM4500 has additional cables / hardware behind the glass panel.