Just curious. You said you have the most current software in your pilot. If so, when the software was updated, did the installer take your boat out and re-swing the electronic compass? The only reason I ask this is it is common to do a master reset of the system after installing new software. If this was done and then compass was not recalibrated, this might be the cause of this 4 degree swing. I'd recommend just trying to re-swing the compass the next time you are out in an open area as you have to turn the boat in circles for a couple of minutes while the compass calibrates itself. To do this:
Press and hold the Menu button down and quickly press the knob 3 times.
You will see a hidden menu pop up.
Use the knob to scroll down to Sea Trial and press the knob to enter.
Use the knob to scroll to Compass Calibration and press the knob to enter.
You can choose manual or auto. The only difference here is in manual you turn the boat to starboard or in auto the autopilot will turn the boat to starboard for you.
Begin making a circle to starboard at about 3 knots. You will see a % indictation appear on the autopilot screen. Keep turning to starboard until it reaches 100%.
At this point you are done. If it passes the calibration successfully, which it should, go run a quick route and make sure the autopilot is functioning properly again.
Please let us know if you need anything else.