Your best chance is probably an open truss framed fuselage and a built-up wing with no sheeting - kinda like a Sig Kadet Senior with a symmetrical wing. Use Econocote to save weight.

Come to think of it, my WM Super Stunt 60 could probably have been built to be less than 4lbs airframe.

You can save weight with Lithium Ion battery packs, but you're stuck with standard sized high-torque servos if you want to 3D.

I think the standard answer project engineers have for their manager applies to this:

I can make it high performance,
I can have it done fast,
I can do it cheap.

Pick any two of the above.