WOW....4 miles?!.....This is a great reason to set an anchor alarm on a handheld GPS that you keep next to your bunk if you are ever concerned that will happen again.
These handheld GPS units are getting cheaper and cheaper BoatUS sells the Garmin eTrex for just $119. Much cheaper than a tow or any other damage you might have incurred.
The anchor alarm can be programmed to trigger on whatever range you set. They are so accurate these days (with the WAAS enabled ones) that if you set it to low it will trigger when the boat just swings on the anchor (So you then need to back off on the sensitivity)