A plant that has many uses - including being
tolerant to salty soils is Sea-Buckthorn (
Hippophae rhamnoides ).
See
http://en.wikipedia.org/wiki/Hippophae_rhamnoides
It is drought tolerant, stands poor soils, fixes nitrogen, is great on steep slopes, and produces an edible berry.
The berries are quite astringent, and often mixed with sweeter juices. Extremely high in vitamin C.
The plant is very thorny, making it difficult for humans to harvest. Birds, on the other hand love the fruits, which stay on the bush all winter (hardy to zone 3). Some consider it to be an invasive, partially due to all of the seeds that the birds poop around the neighborhood all winter.
The plant also has many medicinal properties, and its high oil content is utilized for many body creams and other applications.
Sea-buckthorn is deciduous, so it will lose its leaves each winter, but once mature, the many branches & berry clusters still make a dense obstruction to both sight and sound. Think a dense row planted on each side of the berm, about half way up from the bottom.
If your property is fairly level, a 4' berm will be just about eye-level for passing motorists.
Anything growing above that is just icing on the cake.