Photo? Or at the very least a bit more info. Any other shrubs or trees nearby and if so, what species? Buildings or pavement close by? Soil type (general pH and texture)?

not many plants stop growing at some magical height .. and anything that grows fast.. to that height.. will probably keep going right past it ...

we do need a pic to really help ...

also note.. most things that get to 8 feet tall.. could chew up that 4 foot space ... unless you are willing to do some pruning ...

ken

Sorry, I'm not picture savvy. As for details, this is a suburban condo. My back garden extends about 18' from the back door then meets a huge, open common area. That changes into native trees and shrubby undergrowth and a creek about 50 yds away. A lovely view. Until next year when the County is tearing it up to put in a 4 lane road!!!! We expect it will still be about 50-60 yds away but with no buffers.

My beds do include trees and some shrubs at the edges. The shadier side is heavily planted but has one 4' gap between a Viburnum and a giant redbud with a very large trunk. The other is on a curve with a small trunked yellowwood on one side, my path on the other and nothing behind that will block the view. This area is pretty open and the tree is about 4' from the path. Both trees were limbed up years ago (for unknown reasons) so there's lots of height available. The soil is heavy clay and acidic to around 6.3.

Yes. I do recognize that the height vs width is a problem. I've been wrestling with that for months. But I'm getting too old to do regular pruning and the most I'm willing to do is hire some idiot with a pair of loppers once a year.

My sympathies. If there is a road going in back there, plants don't provide a sound barrier, only a sight barrier. If you want some sound protection, you want a solid fence between the fence and the road. Can you get the condo association to put in a fence? Or do you have the space and the $ for a fence?

Any plants you decide to use will need to be staggered some from the trees since most plants will have difficulty 3' from a tree trunk and you are likely to damage tree roots in planting. Look at some of the arborvitae cultivars. See what you can find as far as narrow rhododendrons, though I am not confident that you will find one 4' wide. Or put in a couple of decorative fence panels or even a trellis or two with a large clematis or honeysuckle vine that's not one of the invasive ones (Skip Lonicera japonica and look at Lonicera sempervirens or Mandarin.)