#1
#2
Speaker cable has a bigger conductor that's meant to handle the amount of power being pushed through an amp. Instrument cable is shielded and has a much smaller conductor, which can melt or separate if used between a head and cab. If that happens it might cause some damage to the amp as well, since it will be operating under no load (or a short, depending on how the cable breaks).
#3
Yes.

Do not run an instrument cable between your head and cabinet. You (or your amp) will die.
Quote by DeathByDestroyr
What the hell is a G&L.



Quote by Flux'D
Gay & Lesbian I think, the box smelled funny
Greg what did you send me??
#4
Speaker cables use a thicker wire and are designed to take the outputs of amps. Using an instrument cable to connect an amp and cab will fry the cable and eventually your amp. Using a speaker cable for your guitar will likely sound choked out, but I doubt it would break anything...
Maximum volume yields maximum goats.