I have both. I've owned the 4 for over a decade and just recently replaced the grips with TKC long grips. It turned a good knife into a fantastic knife. I've skinned and butchered many deer and small game with the knife along with using it for hundreds of cutting uses over the years. Just a solid choice.
I talked myself into ordering the 6 a few months ago after watching tons of YouTube videos which got me interested and then researching on a bunch of other websites. I ordered the new contour natural color grips model. Everyone seems to think it's the cats meow. I think it's way too damn big personally. When I pulled it out of the package my first thought was "this is one big ass knife!" It's too small to cut firewood other than really small kindling stuff. Sure, you can split kindling all day with it but that's the extent of its usefulness. It's too thick to be a good slicer as I've tried it with a ton of fruit, vegetables and meat. Too long and ungainly for peeling. It looks really cool. I'm just at the point where I'm asking "why did I order this and what do I do with it now?"
If you want a good suggestion on something completely different, I can highly recommend the Buck-lite maxII large. I picked this up recently and I don't know where it's been all my life. Seriously love this thing for a utilitarian fixed Woods blade.
BuckliteForgot to mention the weight of the 6. It's way heavier than the 4. This is a seriously big and heavy blade! Definitely check it out at a dealer if you can and compare the two. That's the best advice I could give you.