One of the guys in my club built a 1/12 DC-3 last year.
He is builds some pretty light airplanes, his with a pair 0f .32's for power flew very nicely at around 6 lbs...
It did however have the classic tip stall/loss of aileron authority when he got too slow. I would say the slowest he could get it for landing was closer to 30 mph. any slower than that was an issue.
Given that you are building it for electric power your framework would not have to be any where as robust as his so if you could get your target weight closer to 4 lbs you may be able to get the performance you want.
As everyone has pointed out your most critical issue will be keeping the wing loading as low as possible.
If possible use laminated balsa instead of ply if you need a bit of strength as far as motor mounts and alike, you would be amazed just how much strength you can add to balsa with some 1/2 oz fiberglass applied with epoxy with all of the excess epoxy blotted off with a paper towel. Carbon fiber veil applied with dope also will add strength without adding a lot of weight for stress points such as spars and shear webs ribs and alike.
Using a very lightweight receiver as well as servos, and opting for pull pull control linkages, rather than rods is also good for shedding an ounce or two.
Using the absolute smallest lightest li-pols for your power requirements is also a must, granted this may limit your flight time but given the high C rating batteries available today this is not as much of an issue as it was even two years ago.
Bottom line get as much air into your airplane as possible!

Good luck!