Cold Steel Voyager 4-inch/5-inch leaps to mind. Strong, light, sharp, well made, extremely depedable.
Cold Steel Gunsite, 4-inch/5-inch. Same as above, but with standard half plain-, half serrated-tanto blade. This is my EDC.
CRKT M-21-04 4-inch weighs in a bit more, but is strong, well made and attractive. Very impressive, but may weigh a bit too much with its aluminum frame.
Kershaw Storm II 4-inch is strong, slim and attractive. Abrasive strip keeps knife from slipping, but may be too abrasive for some. Recurved blade may turn some people off. Excellent build.
Cold Steel Spartan. With 4.5-inch blade, this is the strongest folder made. Robust, large and somewhat heavy, this knife rivals fixed-blade knives in strength, and is better suited for heavy cutting and outdoor use.
Ebay is the best place to pick up these knives.