Well,
I think this is one of those "win-win" situations, as in, no matter which one you decide on, you'll get a darn good knife! It is also an "apples and oranges" comparison, however, as each knife has its own unique features.
The Military, which is significantly more expensive, is commonly thought of as being one of the most reliable, best performing, liner-locking knives around. With a 4-inch, fully flat-ground, S30V blade, it cuts like a laser! The Military is considerably light for its size, this being due to the nested liner and G10 handle. It also has a 50/50 choil for detailed cutting.
The Endura (IV) is a Spyderco best seller and is a testament to Spyderco's constant desire to improve and innovate. Spyderco uses their very strong and reliable front-lock (lock-back) on this model. With a 3 & 3/4-inch, saber flat-ground, VG-10 blade, it'll also keep on cutting. The FRN handle slabs contain nested steel liners for extra strength, and the FRN, with the improved, bi-directional, volcano grip texturing, is quite grippy.
I'd have to give the edge (pun intended) to the Military in terms of slicing, due to its fully flat-ground blade. I think the Endura wins out in terms of blade durability, due to its saber flat-grind and re-inforced tip. I have yet to notice an appreciable difference in S30V and VG-10 in terms of edge retention, but VG-10 sharpens a bit easier.
I generally prefer G10 to FRN as a handle material, however, Spyderco has made the texture on the Endura quite grippy. Due to the fact that the Endura has nested steel liners, I think the strength difference between the two models' handles is pretty much non-existant. I do prefer a lock-back (front-lock) to a liner-lock, but in the case of the Military, the liner-lock is really very reliable and strong. The 4-way clip on the Endura does, however, trump the right side, tip-down only clip on the Military
I think it really all boils down to how you plan on using the knife. Are you right-handed? Will it be a beater? Will you do a lot of slicing? What are the blade length restrictions where you live and work? If you lose it, can you afford to replace it?
Hopefully, I have given you at least a little bit of help in making your decision and have not confused you further. With Spyderco, it is always tough deciding between models, especially when you are considering two of the very best! Good luck!
Regards,
3G