a) First off, digital recording via software and line in/mic input is the easiest and by far cheapest option. So i would start with that.
Analog is still preferred by some, but most pro systems are a combination of both, it really depends on how serious you are about recording.
b) Mixing is done via a mix desk, not the physical 4 track itself, some do, some don't. You could record onto the 4-track, interface into a program like cubase, pro tools, or likewise, and mix digitally.
c) It really depends on your model again, and which method you would rather. Again the output can be both digital or analogue depending upon your model.
d) refer to c)
e) Digital interfaces usually have firewire/IEEE or usb interfaces for fast transfer.