Rhamnus californica 'Mound San Bruno' is a great evergreen shrub that can get a bit taller but can easily be pruned to shape/size. Nothing flashly, just a nice, attractive broadleaf evergreen. Inconspicuous flowers.

Arctostaphylos 'Howard McMinn' is the most garden-friendly of the manzanitas, is about the right size for you and has lovely flower display in winter. Inner branches die but can be pruned out to show off twisty, cinnamon-colored bark.

There are a number of Ceanothus that would fit your requirements but they are short-lived and not particularly attractive when not in flower. 'Dark Star' is one of my favorites.

Lots of the other California native shrubs (Garrya, Myrica, Heteromeles, many manzanita) get much bigger that 4' tall.

Callistemon 'Little John' is not a native but is drought-tolerant and quite attractive. Check hardiness for your zone.

You might consider a Leucadendron such as 'Jester' or 'Safari Sunset' if hardy where you live - they are attractive year-round.

Lots of small trees have shallow root systems so would not cause problems with your pipes.

Good luck!

Sara

those are all very good suggestions.

Thanks for the great suggestions! You mentioned shallow rooted trees. Can you identify which I should be looking into? We are looking for height in that area but have written off trees out of concern for the plumbing issue.

Most small trees are pretty shallow rooted. You might check in with your local nursery if you have a decent one around that has trees or even better, specializes in trees. I think you'd be ok with crape myrtles, crab apples and the like. Also, if you identify a few specific trees that you like, try Googling that specific tree with the words 'root depth' or the like and see what you find. Sites posted by Universities, etc are generally quite reliable.

Have fun!

Sara