We too are zone 4,maybe 3, Sandy soil. Lots of good suggestions already. I like to plant lingnonberries between blueberries. Both like acid soil. I've got cranberries or Nagoon berries by the honey berries, garlic ( the kind that gets the bulblets on top like Egyptian onion) they will spread like crazy tho. I love purslane salads and you can order "improved" varieties that are a little bigger leaves, look like a jade plant.violas,violets are edible. You can tuck cucumber seed in and let it crawl. I've been reading about growing wine cap mushrooms in straw or wood chip mulch in garden beds, tolerates sun good luck let us know what works for you