Just to clarify why a 4 is not a big deal:
Below the poles, a K index of 4 will give you mild ionospheric delay which changes slowly over minutes and hours. If you were to plot an absolute GPS position during a K index 4 event, you would likely see horizontal loss of precision increase by an average of 2 - 3 meters and it might shift a little more quickly than your regular ionospheric drift.
But, this will have very limited impact on relative positioning during a flight lasting less than 20 minutes. So unless you're trying to drop a photon torpedo into the ventilation shaft of the Death Star, I think you'll be OK at 4.