[Varyon]

Another point of note - those rockets used in Mythbusters (and the alleged Korean device) aren't the explosive rockets you see in Low-Tech (and the video I linked), but rather more like rocket-propelled spears. Those are likely going to be a good deal more aerodynamic than the explosives, and without the heavy weight of an explosive payload are going to be more predictable in their flight, but ultimately aren't much better than a bunch of guys throwing a volley of spears or firing a volley of arrows (they might have better range than the former and better damage than the latter, and they've got the advantage of needing fewer people to handle them, but they'll reload more slowly and be much less accurate).

EDIT: I should also note that modern black powder is likely a good deal more reliable than the corned powder of TL 4. It's going to be at least as good as the improved corned powder of TL 5, and likely a good deal better.