As a tangential comment Eikadistes, I also perceive a tendency in the "fourth leg" argument to conclude that the assignment of a word to a particular thing (the grasping part, i gather) involves a little more steering by nature than I think is consistent with Epicurus.
As I read the discussion of language and civil society in Lucretius, it seems to me that the real stress is on "these developed naturally rather than being given by supernatural gods," rather than "nature leads us to associate certain words with certain things" or "nature leads us to a proper word choice" or "nature leads us to a proper system of government." I see both those as a "trial and error" process in which there are lots of different languages, and lots of different systems of government, that can all be equally consistent with "nature."
I have a lot of respect for some who argue that there's a fourth leg, and clearly Diogenes Laertius says that "the Epicureans generally" (I think is the phrase) embraced the fourth leg. But to the extent that these other Epicureans deviated from Epicurus I think they were mistaken in doing so. The whole thing sounds to me like an improper attempt to reconcile with Stoicism.