I still don't know what a full number system is, although you and nydrwracu refer to it again. Is the claim that English has one but Proto-Uralic didn't? If so, how is the distinction drawn?

The case of the Pirahã is different. They have less of a number system than the rabbits of Watership Down, and less of a number system than has already been established for Parseltongue. It makes sense that they couldn't learn to count [although the children could, if I remember correctly what I've read about them]. But I find it much harder to believe that a culture that can count to 4 can't learn to count beyond that.

As for confusion, I'll buy that you get confused much earlier if you grew up counting to smaller numbers. Most English speakers have no good idea how big a million is, even if they're comfortable with the word. Nobody has a good idea how big 3^^^3 is.