Answer: When planting a hedge, there are several considerations to keep in mind. One is that if all one type of plant, that plant will grow differently in different conditions. So if you use a sun-loving plant, the portion grown in less than full sun will be sparse and may not thrive as well as those growing in their preferred sunnier location. The initial planting will take years to grow and mature and meld together to form a solid barrier, so there will not be an instant solution to the problem. The hedge can be one that is formally trimmed/clipped or informal -- where the plants are allowed to meet at the tips but grow to their full size. If the hedge is kept tightly trimmed, you may sacrifice fruit and/or flower production as a result of the constant trimming and the trimming is an ongoing commitment.
All in all, you may find that a fence is a better solution, you could use clematis or a few roses to soften the look of it. But if you prefer plants, you will need to analyze the growing conditions and make a selection based on what will grow well there. Sun, wind, soil type, and moisture levels are all important considerations.
In my experience pyracantha is not very satisfactory in northern areas where winters are cold as it tends to defoliate in zones colder than about USDA zone 7. If you want a broad leafed plant and the site is sheltered from the winter wind, you might be able to grow a rhododendron (select a cultivar that would mature at about four feet to avoid trimming) or possibly one of the relatively hardy blue hollies (Ilex x Meserveae cultivars.) These eventually exceed your four foot size but will tolerate clipping and are hardy into zone 5. You could also consider something like an arborvitae or even one of the mid sized junipers. Rugosa roses would also be a possible choice, although roses as a rule do require a minimum of six hours a day of full direct sun including the hour of noon. (The rugosas as a group are very hardy and low maintenance roses.)
Your local professional nursery staff should be able to help you analyze the growing conditions where you want to plant your hedge and suggest plants that will thrive there, then you can select the ones you like best. You can certainly mix types, but be sure to allow adequate space for each type to grow over time. You might also want to look at some mixed plantings to see if you like their overall effect. Good luck with your hedge!
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