I'm still gonna say, 9 if you want a chopper (and that's one that you just need). 5 if you want a camp knife or a great slicer. A tweener if you want something generally useful (and if the 5 is interesting, the 15 would fit the bill nicely for dual purposes). RBK if you just want sheer bada--ery (or maybe a Brute, or a 6). Or a TMHunt M-18. I kinda want to try one of those out, TBH.
Personally, again, I'm utterly uninterested in the 4. It looked so much better in pictures than in person, in my opinion. I love curved blades, BUT once I got it in my paws, I just didn't really like it. I think it might be the only Becker I've sold. YMMV though. It's clearly got fans, and it's a perfectly competent blade. I'm not sure why I didn't like it, honestly. I really feel like I should. It might be that I like wider blades, and the blade on the 4 is quite thin width-wise. I was actually kinda surprised. Pics make it look bigger than it is.