Someone suggested the Spyderco Military. Great knife, I bet...but doesn't meet the 4" requirement. It's 3 7/8". But if you're not THAT picky, I'm sure you'd do well buyiong this piece.
I don't own a piece over 4"...in fact the largest folder I own is a CRKT KFF (3.87"). However, from what I've read and learned, seems like the SIFU is your best bet. It is the only "megafolder" out there, and possesses a lock considered superior in strength than most (rolling-lock).
HOWEVER, this piece comes with a whopping 5.4" blade, and overall length comes to 12". So, unless you don't mind its huge size, I'm sure there are many great 4"-5" folders. The thing I find dissappointing is that none of these possess an Axis or Rolling lock. I don't really know much about 4"-5" knives, since I rarely look at them. I have no need for them and they would only attract trouble if I carry them. I always prefer and stay between 3"-4" blades.
So, in conclusion, I would get this knife... however I have no use for such a big knife. I would never attempt to carry it, since there is no way I can get around legal hurdles. Plus, you can do most things with a 3"-4", including self-defense.
Hope this helps,
-Flood
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HERMES: "The Applegate Fairbarn is not bad...but for both offense and defense, the Randall 14 is good."
HERMES: (Admiring the Joe Kious Sub-hilt dagger) "I'll take the Joe Kious for a friend."
KNIFE-SELLER: "Excellent choice."