I think the features are a matter that you have to work out. If you're looking for a "working knife", you'll be most interested in ergonomics and edgeholding. For a strictly defensive folder, you're interested in the best lock and smoothest action available.
My vote, in both cases is the Sebenza. The lock is simply second to none. You can legally carry on a plane. It holds an edge extremely well.
Drawbacks: Ergonomics (as with most pocket clip folders) is so-so.
Blade legnth is just under 4". (This is perfect, IMHO, but slightly outside the range you asked about).
Highest recommendation for it comes from Kevin Mad Dog McClung. Kevin makes the finest fixed blade tools and weapons available. When his son went to college, where fixed blades ar a no-no, Kevin gave him a Sebenza.
Hope this helps.