I must say, my endura3 is the single knife I've abused/used the most without "any" problems, and I've used the hell outta it. For the price, if that's a concern, get one. -But, my endura3 has no liners, and, though, perhaps, in the minority, I perfer the 3 to the 4. -Liter, smoother, pointier. -No need for the up grades. When I first held the new 4, I just had to have one. -Totally upgraded. But, those upgrades were totally unessessary in my opinnion. To me the only new feature I really care for is the thumb grip serrations. -But that's just me.
I, would consider how "tough" of a knife you realy need to have. Toughness is great, but comes at a cost. You may or may not want to consider ease of carry (weight/thinness), and ease of sharpenning/edge retention, (steel). If you plan on stabbing or poking/using the tip, with the knife, you may want to see the 710(axis) as the best option.