I would like to create a formal English garden look, where “squared-off” hedges line the garden perimeter. Can you recommend hedge plant options (other than yews) that grow at least 4 to 8 feet tall, look thick and full (like a solid wall), and grow in sun and shade?
You have several restrictions in your plant request that limit your choices. Your height requirement creates a problem for boxwood (Buxus sempervirens), a favorite of formal English hedges, because they are a rather slow grower. However, they hold the geometric pruning well, are sun and shade tolerant, and form a dense thicket of glossy leaves. Boxwoods are the choice for knot gardens, parterres and topiaries because they are evergreen and hold a crisp line when pruned.
Privet (Ligustrum amurense) grows quickly, is easily sheared to shape and can reach your desired height. It is also tolerant of light and soil variations. Unfortunately, in zones 3 to 7, it is considered deciduous, even though its dense twiggy nature is a screening element in itself.
Since the perimeter of the garden is the designated planting zone, you might consider Japanese holly (Ilex crenata), which is very shearable and has a medium growth rate. Their leaves are spiny but lustrous. They will tolerate full to part sun, but need somewhat acid (low pH) soil and good drainage with sustained moisture. The downside is that they need protection from drought and wind.
There are few if any pines or spruces that meet your criteria. However, the workhorse of the residential landscape, the columnar American arborvitae (Thuja occidentalis), may suit them all. The varieties ‘Emerald’ and ‘Techny’ both meet your height requirement of 4 to 8 feet. They hold their color in winter, and have considerable heat and light tolerance. Shearing and pruning them before spring growth will keep them full and dense. They have a natural pyramidal tendency, but can be nicely shaped to be wider at the base, to give the “squared-off” hedge look you are striving for. Keeping them slightly wider at the base allows light to reach the interior of the shrub, allowing active new growth and preventing thinning of the interior.