If you can control the torque of the K4, and learn to 30mm (which never seems to want to shoot where you aim
) then the K4 is just as easy to fly as any other plane. If any of the 109's could be considered "easy mode", then my vote would be the G2. It's maneuverable, quick, fast, has a decent climb rate, and doesn't have the massive torque like the K4 and the 20mm is far easier to aim.
No plane is invincible. You can use any plane to kill any other plane as long as you use your strengths against your opponents weaknesses. That being said, no plane may be invincible, but a good pilot can make it seem it is. So get to reading up on ACM and start getting better! Instead of focusing on flying the one way you're good at, focus on the things you're not good at and get good at them. That's how you make people say "Dang! We can't shoot that guy down!"
15 ENY seems fair for the K4, but I've flown it so much that it just seems easy to fly. I think the torque, 30mm, and the fact that it has no ATG abilities are what puts the K4 at 20 ENY.