Ego Death
Just wondering if anyone can tell me. I'm talking about folding knives. Only fixed blade I'll ever need is my KABAR.
Some options I can think of would be, Any of SOG's SAT assisted folders including the Aegis, Flash and Trident. The Aegis being my favorite. They are all around 3oz with a 3.5 inch blade. Keep in mind they are linerless but the GRN(Glass reinforced nylon) handles are plenty strong.
There's the Benchmade 530 which is an astounding 1.9oz considering the blade is 3.25 inches. It has partial steel liners but the GRN handle is quite thin. It's easily in my top 5 favorite folders.
The SOG Pentagon or Spec Elite are fair options as well. Although at 4.3oz they are not exactly light weight but they do have 4inch blades. Be prepared to spend around $100 though.
You might want to check out the CRKT M16-13T. Because of its titanium handles it weighs just 3.3oz with a 3.5 inch blade. In fact most the m16 series knives have agood weight/blade ratio because of the extensive skeletization(multiple holes are drilled out of the handles and liners.
Here's the CRKT 13T. Man it's purdy.
sweatdrop 
There's the COLD STEEL Ti-Lite in GRN handles and full steel liners. It's a super strong knife and weigh 3.5oz with a 4 inch blade.
There's the $150+ Benchmade 940 or 943 both blade lengths of 3.40" and 2.9 and 2.65oz respectively. The price tag is due to the outstanding S30V blade and color anodized titanium back spacer.
There really are many more options. Those are just a few of the best IMO.