I believe in chopping the BK4 and BK9 are about the same. But each one has it's own advantages that one needs to consider before buying:
BK9; tradition blade shape, makes a good improvised chef's knife, The point is inline with the handle which everyone is used to.
BK4; Makes a much better draw knife, pretty curves, you can sharpen different sections of the blade for different uses (recurve thin for slicing, draw knife, main curve thick for chopping), better at hooking light vegetation for cutting. In the right hands, a blade with this shape has the potential to be a more effective chopper, brush blade then a standard straight blade. It has something do to Geometry, force, leverage and such.
I have a Camillus made BK9, SOG Tigershark (First Gen, Carbon steel blade), Kabar Large Heavy Bowie, and a Fiddleback Forge Camp Knife so I have experience with Bowies. I also have at least 6 H.I. Khukri's and over 12 Machetes. For lots of chopping and brush work, I use my Khukris and Machetes more then the Bowies/camp knives. . . . For me I'd lean more towards the BK4 Machax, it's more Machete/Khukri like.
My woods combo always includes a Machete or a thin, fast and light Khukri. Most everyone knows how good a Machete is when it comes to brush work, but what they don't know is how good of a Chopper the larger Machetes can be. My 18" Bladed USGI Ontario Machete is one Mean chopper!! Years before I got mine, I used a friends to clear some trails durring a service project. Between it and my 20" Sirupati Khukri we had the brush and trees all taken care of.