zone 4 are very cold the temperature can go down from -25 to -31 Celsius or -20 - 30 Fahrenheit. A zone cllimatica frozen for our Italian latitudes. There am Opuntie that can withstand those temperatures but I would not exactly know what. It will be important however that the plants are completely dry and protected at least to the base with dry leaves, straw or other materials. I suppose that to those temperatures there is little damp and this is very good. Other I would not know how to recommend you
