I had read that you can fill some plastic barrels with water, the sun heats them up during the day and then they give off the stored heat all night, so that's something I would try to keep a green house warmer at night. My use for a greenhouse in zone 4 would be to move my started plants out to it in March or April, because most can't be planted until June, and then I would use it in late fall to get a few cold season vegetables, until they finally froze.
I start my seeds in 4 inch pots, fill with soil, pack, wet down with hot water, put seeds on the surface, water in with warm water, cover with plastic wrap, rubber band, then put in a sunny window, after they develop true leaves I transplant as many as I need into cell packs, then they go under lights with a fan blowing on them for a couple of hours a day.
Years ago I went to the library and got a book on solar greenhouses. They had a greenhouse in Wisconsin that was supposedly heated with a pool of water in the middle of the house. They claimed it kept the house above 45 degrees. I have water in both of my houses. I filled Pepsi cans with water and stacked them up for shelves. The problem I have had lately are the cold sunless days. The water cools and refuses to heat up much. I have a livestock tank with water in the other house and when we get those sunless days I drop a bucket heater in it at night. I run a propane tank in my bigger house and use a homemade heater. I can run it low enough to get several days of heat, day and night, to get by. I also have a couple of kerosene lamps for the small house.
It is supposed to get down to 28 degrees tonight and then the lows will be above freezing for the next couple of days so we don't have the very cold weather you get up there. At 28 degrees I could use heat lamps and probably be okay but I broke my last one a week or so ago. When I get some cash together I am going to probably go with kerosene and a fan.
My next house is going to be all solar. The house will face east to west and the north side will be insulated. I am going to use water tanks for heat and raise fish in the tanks. Water takes a long time to heat up but it also takes a long time to cool. I think it is better to get your water heating in the house during the summer months.