Hence is a lot like "therefore." I'm not sure about the four meanings, but in the examples above they all seem to mean the same to me and "therefore" could be sort of replaced for "hence" in each of them. (A) Icahn’s gambit clearly rattled Netflix, [therefore they used] the poison pill defense. (B) [Therefore], when it comes to increasing sales, words like ‘problem’ are verboten. (C) He'd finally showed some backbone, [therefore] some pride. You could also use things like "as a result," "consequently," "thus," "wherefore." http://thesaurus.com/browse/hence?s=t "Hence why" I've heard before but I wouldn't say it's "common." To be honest "hence" is a bit of a fancy word that I see a lot in writing (and maybe hear in formal/professional situations) but don't hear too often in general conversation. =)