I'd go with the 4. It was my first and I wanted to a good general purpose knife that was easy to control for finer work, but could be used to baton firewood if need be. It is light weight, comfortable to carry on the belt and is a great blade overall.
I have a 6 that I really like, but it is not a chopping blade and if my priority were processing of firewood, I would carry a hatchet. I find it to be a little light for chopping as most might agree.
With that said the 6 is a hoss of a knife and if you think you would have more use for a larger blade, the 6 is a great contender.