Ask Mr. Smarty Plants

There are several narrow shrubs that would be good informal hedge plants for your sunny, dry site. Since you asked for plants that are less than four feet wide, I am assuming you want a hedge that is about that height too. The first place to go to find a list of potential hedge shrubs is our Native Plant Database.  Use the Combination Search feature instead of Recommended Species. This will provide a bigger selection with much more choice to narrow down. The volunteers and staff at the Wildflower Center who maintain the database have partners in different regions to help with these recommended species lists based on what is easy to access in local nurseries.
Under Combination Search, select the following categories: MA, Habit – Shrub, Duration – Perennial, Light Requirement – Sun, Soil Moisture – Dry, and Size Characteristics – 3-6 ft. You can narrow down this search further by indicating, blooming time and leaf characteristics.
Follow each plant link to our webpage for that plant to learn its growing conditions, bloom time, etc. At the bottom of each plant webpage, under Additional Resources, there is a link to the USDA webpage for that plant. Take a look there for more specific details about suitability before you put them on your final planting list. Think about including plants that have interest during a variety of seasons and that have more than one attractive feature (flower, fruit, foliage, bark, etc.) so you can get more benefits out of your hedge plants.

Here are some informal hedging suggestions from the Native Plant Database search. Most of them will just require a little judicious pruning of wayward stems to keep them within your width requirements.

Rosa acicularis (prickly rose) is a shrub to 4 ft tall with single pink blooms.  The fruit turns bright red when ripe later in the season. 

Rosa blanda (smooth rose) is a bit taller (to 5 ft) and has pale pink to white blooms. It is a dense, mounded shrub and spreads vigorously by root suckers once established.

Amelanchier stolonifera (running serviceberry) grows 3-6 ft tall and has prolific white blooms in the spring. Edible berries ripen in summer and are loved by birds and wildlife.

Ceanothus herbaceus (redroot) grows to around 3 ft. Small clusters of white flowers bloom in early summer.

Dasiphora fruticosa ssp. floribunda (shrubby cinquefoil) is a rounded shrub to 4 feet in height and width. It has long blooming showy yellow flowers during the summer.