Does it get sustained low temperatures like that? Or just like a morning and then warms up? They live in many areas naturally that freeze. 3-4c is above freezing so should be fine unless it’s months at a time. Gar will lie dormant on the bottom and not move for weeks when it gets really cold.
I would introduce in early spring so they have more time to get acclimated before winter. Also, 30inches or deeper would greatly help with extreme heat and cold.