Doc: I hear you on the "little short on jungle" part. I'm sure I don't have nearly the experience or skill with any of these blades as you or a lot of other folks here, but for what it's worth....
I have a few KB Machaxen (one of which I modified and it gets the most use), a 20 and a 21. WAY more dirt time with the 21 and 4, as the 20 showed up a week and a half ago and we've been having some rain. Being in NH and inhabiting mostly hardwood forests, I would guess we have similar environmental conditions. Here's my take, and it is ONLY MY OPINION - and based on some limited experience with the 20. IMO the 20 is a better pure chopper than the 4, mostly because, in my experience, the curvature of the 4 is limiting on bigger stuff. The 20 has a bigger sweet spot. However, the thicker blade and more even weight distribution don't give it the "snap" of the 4. I don't feel that it has as much versatility (again, for me....and my limited experience with the 20 so far) as the 4.
I'm sure that we have similar environment conditions.
I also think the RBK (21) blows the doors off the both of them for chopping, though it's weight can be fatiguing if one "forces" it, as well as controlling it for fine work. Don't get me wrong...I like the Bundock; but for just chopping (and some limited fine work) I'd carry the RBK. For all-around woods bumming I like the Machax (or my trusty BK12) for it's utilitarian size and weight, as well as its versatility.
I know what a Machax fan you are - I personally am of the opinion that, while it's a fine blade, the Bundock isn't gonna blow your mind. I could be wrong - I often am.
That's something we share.
I own a few choppers and, while I most certainly need more time to acquaint myself with the Bundock, it isn't knocking my socks off in the way the 21 did (and still does). There'll be some that come on the exchange for reasonable $$ - even $CDN - and there is one on there right now, I think (= to just over $200 CDN). Your love of the 4 would indicate to me that you might want to check out the 21 - mor kuk, less machete....and wickedly effective on hardwoods like maple, birch and beech.
As I mentioned above, I'm sure the 20 would outchop the 4. I got to thinking. I really don't canoe camp or car camp any more, so the only way I would be using the 20 or 21, for that matter, would be, to use your terms, for woods bumming and they just seem too heavy for that. Heck, by the time you take all your toys in your pack, along with a few beer (BTW, we don't normally drink bottled beer in the woods, but we needed it for the BK-11 comparison) the darn thing starts to weigh a ton
I also noticed when checking my blades, most of my larger blades, except for a pair of 20" Martindale machetes, tend to fall around 9" in length (3 BK-4s, 1 BK-9, 1 HI CAK, 1 HI M-43 and 2 Linder short machetes with 9-1/2" blades. It seems to be that this length works best for me. Heck, I don't even do that much chopping anymore, so I think I've decided. I just wish they'd sell out and remove the daily temptation.
The downside is the 21 sheath, IMO. The standard nylon sheath that comes with the 20 is fantastic in comparison to the RBK's sheath. Still, it's a downside I can live with.
Actually I'm pretty easy when it comes to sheaths. If it protects the blade and my skin, I'm happy.
Anywho, good luck with your decision making. Hard not to jump on the bandwagon when Ethan and KB introduce something new, especially when it's an LE. I wasn't going to get one - but I caved....which is a lot easier when I don't have to deal with an unfavorable exchange rate. Still don't know what I'm going to do with the other 2 knives that came with....