the 3rd generation one does not have steel liners, the handle is only plastic while endura4 is made with steel liners for added strength. endura3 has a finer tip, which is pointier while endura4 has a more "robust" tip to decrease the chance of the tip of the knife chipping or breaking. its a trade off. some people seem to like endura 3 cause its a bit lighter and has the finer tip while some prefer endura4 for the upgrades.
endura 4 is put together with screws while endura3 is made with pins that cannot be adjusted.
i think thats about it.
there is also the spyderco salt series which is an endura3 handle with a similar blade as the endura, but its made with h1 steel so its rustproof and harder than vg10 if its fully serrated.