You have cold winters. Seeing as they are both temperate to tropical fish, they wouldn’t make it a whole winter at 4 deg. celsius. I don’t think they can adapt to ”shut down” like most cold climate fish do during the winter. Also, an outdoor pond will need to be pretty deep in cold places to avoid bottomfreezing.
The depth doesn't matter much when it comes to temperature, more so when it comes to freezing. If it's a deep pond, that just means there's a body of water large enough that the outside temp can't consistently cool the water to freezing temperatures. As soon as the water reaches 4 degrees celsius, that water will "sink" to the bottom because 4 deg. Water has the highest density. So, the bottom will always be 4C if the water is cooled down that far.