By all means, try zone 4 plants. Rating plants based on hardiness zones is not an exact science, and gardeners push the boundaries all the time. When growers introduce a new plant, they often send samples to gardeners in different hardiness zones to test and define the boundaries. They might, for example, have a 50 percent survival rate in zone 3 and then label the plant for the warmer zone 4 so that half of their customers in zone 3 won't suffer losses. Dabble with zone 4 plants but know that you're taking risks. Some plants might surprise you and do extremely well while others might not survive the winter.