there are many interesting xeric plants offered at High Country Gardens - a lot of them are zone 4, but only for the dryest climates. What does well in zone 4 (Iowa) with a minimum of care and watering? Maybe we could start a list. Here's what I grow in a spot that seldom gets watered by anything but natural rainfall (which is a lot more abundant in Iowa than the desert.) I am grouping plants that need more watering in beds closer to the house, and those that need little or no, in areas further from the hose.
Sedums of all kinds.
Tulips like to go dry in the summer,
as do Iris
Comfrey never gets watered by me