I've been using the Ardupilot with Arduino 0018 version code. This allows you to use attach more than 2 servos using servo.attach and writeMicroseconds commands. I've tested it with 3 outputs but 4 should work fine.
I also modified the IMU to write "sentances" of output i.e. GPS data and attitude data on separte lines like Ardupilot is used to.
If you use the code from Ardustation, you can get ArduIMU to talk to Ardupilot this way. The stabilization and navigation code becomes much simpler
No progress yet. No time for hobby. Sorry. :-(