I don't own a 6, but everyone who does seems to rave about them. Additionally, excluding the 5 and aforementioned 6, I do own every other fixed blade they currently offer up to and including the Lite Machete.
For me, the 4 is a little thick for its length, which also adds weight. It does add strength, so if you put a high value on toughness, this could be a plus for you. It's not too thick that it doesn't perform, but the 1/8" thick 3 is much better with wood and slicing tasks. Everyone is different, so comfort is subjective, but the scales on the 4 seem thin, square, and short for me. For this reason, I don't find the 4 comfortable for extended use. Again, this is subjective, as there are people who don't have an issue with it. Also, since the choil blends through the ricasso and straight into the cutting edge, I don't care for it.
The Laser Strike, being the same .188 thickness as the 4, seems to balance a little better. It's 1" longer overall, and a little over half of that is in the grip length, which makes the LS much more comfortable for me. The scales are also thicker, although they still remain blocky. I haven't had any extended use hand fatigue issues with the LS like I do with the 4, and 3 for that matter. Last but not least, I like and use the choil on the LS.
They both come with excellent kydex sheaths, although I think I read somewhere that they were changing some, if not all, models to injection molded sometime this year. Might check over at the ESEE forums for more info, and ask your retailer before you buy if this is deal breaker for you.
If by "do it all" you mean one tool option, with no large chopper, axe, or saw, the 6 or LS is a better choice. If you mean one knife but still have other ways to process wood, ie axe and/or saw, the 4 would be fine if that's your preference.
You can tell from the photo that I didn't use the 4 much, its now "my wifes" knife, and you can imagine how often that gets used. In the end, you can't go wrong, and you'll likely buy more ESEE's anyway.
