i'm having the same problem with my .40 4
i re enforced the afrt of the fuselage and extended the rudder even.
The engine i'm using is a brick.lol

I'm going to install the battery pack behind the servos mount.
a flat pack will fit under the rods.
I don't have a problem with mounting the pack in the canopy..I blacked out my canopy
cut off some the engine mount.
move the tank back as much as i can.
I installed lighter wheels already.
if need too, i'm going to drill ligthen holes in the ply pieces wing tips.

last resort, i would add weight to the tail.
the 4 * .40 is light compare to my other planes. Adding a little bit of wieght won't
hurt it much. Might even help it cut thur the wind better.