Corokid,
Personally, I like the 10x4 much better than the 10x6 on a bushing 40 engine. It gives you much more precise control of the prop thrust.

As far as the flat spin, I'm assuming that you mean an inverted flat spin.

If you give the PBF full rudder while hovering with the elevator near neutral, it will often enter a spin.....sometimes upright.....sometimes inverted.....but usually not a flat spin. To get out of a normal upright or inverted spin, follow the spin recovery techniques on my PBF web page.

However if you hover the PBF nearly vertical, then apply full right aileron control and a just a little bit of up elevator, the PBF will often enter a very flat inverted spin. To get out of this, neutralize the rudder and aileron, then quickly apply full power and full down elevator, making sure that the aileron controls are neutralized. Your 40LA with a 10x4 should have plently of power to get the PBF to instantly exit the flat spin and enter a tight outside loop. After flying through 1/2 of the outside loop, pull back on the elevator stick to get it flying right side up again.

This sounds much more complicated than it really is. And once you figure out how to enter and exit inverted flat spins on command, they are a lot of fun!.....especially trying to see how low you can go and still recover!