Are you 100% sure none of the walleye were in the 4" to 5" range when you stocked them? Walleyes typically reach spawning size around 15"+ at 2 or 3 years of age. The odds of the walleye even spawning in your pond is slim, them growing from 6" to 15"+ from Oct to when they would spawn in the spring is even slimmer, then have YOY walleye reach 4" to 5" by this time of year in your part of the country is pretty unlikely as well in my best educated guess... Could you have mistook some YOY YP for walleye? Could a neighbor have stocked some walleye without your knowledge? When you go fishing again, could you take photos of some of your fish to post?