You'll want a bigger liner if you want a pond thats 4 ft deep. I'm in northern Illinois, My weather gets to -20 in the winter. My pond is 2 1/2 feet deep at the deepest part. A floating heater keeps mine from freezing over in the winter. All you need is a hole kept open in the ice to allow for gas exchange. You could also build a cover for the pond for winter like an A-frame style to keep to snow out of the pond.
Here is a liner sizing tool....
http://www.pondliners.com/liner-sizing.html
Here is a liner sizing tool....
http://www.pondliners.com/liner-sizing.html