4-6 lbs density is "light", and not necessarily "soft", but usually is. I have sheets of different GRAIN wood that weigh the same but the "C" grain is "harder". "A" grain is not as stiff, will bend much easier, "B" grain is in between, and "C" grain is cross grain which is the stiffest.
For fuselage sides, ribs, flaps, and Stab/Elev, "C" grain is usually the choice.
Blocks, and such, are nice if they are "A" grain since they are easier to carve if "A"!

Bill <><