Why does an 802.3 (Ethernet) frame have 2 MAC address fields while a 802.11 Frame has 4?
In classic Ethernet, any kind of repeating or bridging is transparent. There is only one network segment, as far as all the stations are concerned.  If packets cross into another network, they acquire a completely new ethernet identity (and so a higher level networking protocol is needed like IP, for global addressing). The old MAC headers are gone, replaced by new headers.