My 4 inch vent stack sticks out the roof about 14 inches. The warm moist air condenses on the inside, freezing it shut on the inside of the pipe... above the roofline. I chipped the ice out with a long rod, poured hot water to melt ice, and then insulated the outside of the pipe to make it through this winter without sewer smells coming through the house. What can I do to prevent this from happening again next winter? Should the vent pipe be longer?

Making it longer may cause more problems as you now have more surface area to get cold. We don't have this issue in Atlanta so I don't know how to address this Minnesota problem.

What do other people in Minnesota do? If no one has a good answer, here is something to consider trying.

Assuming that you have an attic. Also assuming that you attic is above freezing. Cut the vent pipe and put on a 90 ell, add 4' of 4" PVC, put on two more 90's and come back to the vent with another 4" pipe. All of this elevated to at least 1/4" per foot grade. Connect this to the vent going through the roof.

The logic being that the cooler temperature in the attic would cause the moisture in the sewer gasses to condensate, turn into a liquid and run back down the drain. The cooler but dryer air going to the outside would be less inclined to form ice.

Really don't know if this would work but it should only cost you about $20 and an hour's work to find out.

Four inch is large enough. Freezing of sewer vents in extremely cold climates is just an eventuallity with no really good solutions that I know of. On thing you could do is shorten the pipe to 12 inces sticking out the roof(as long as that is higher than snow cover on roof). In my area (Montana, nice and cold with lots of snow) the minimum height of pipe avove roof is 12". One thing we do from time to time is heat tape and wrap with insolation the vent in the attic. This keeps the vent warm enough in the attic to keep the portion above the roof thawed. I have also heard of people wrapping the vent in the attic with an electric blanket. I have also actually seen people run a length of heat tape up through the roof jack(seal it well with tar or silicone) and stick the heat tape right down the pipe a couple of feet. These are just a couple of options to consider. I would love to invent some sort of solar powered vent top of sorts to eliminate freezing of vents.