Econokote, Towerkote, and Solarfilm are all lightweight coverings, somewhere in between Monokote and tissue as far as strength and weight are concerned. Since there is no fuel residue, they don't need to be fuelproof.

Duration may need to be sacrificed in the name of weight savings, if 3D performance is one of your requirements. If you can save a few ounces by cutting your battery weight in half, and live with 10 minute flights, you'll be rewarded with improved performance with no perceptible loss of power.

Now to the power system... 4lbs... For 3D, 125 Watts per pound is none too much, so we need to shoot for a minimum of 500 Watts, maybe more. I would recommend a 4S LiPoly of some sort, drawing at least 35 Amps, maybe 40 Amps.

The problem with AXis in a .40-size 3D plane is, the 2826 series is too "hot" to spin a large enough prop, and the 4120 is too heavy. Much as I hate to admit it, there isn't an AXi solution to every problem

A lightweight geared motor would, IMHO, be a much better choice here. You can gear the bejeezus out if it and spin a much larger prop than you could hope to turn with an AXI, and gain a whole bunch of 3D efficiency there.