Q.What kind of 4′ tall, erect ornamental grass for zone 4b, south sun, poor soil? Can adjust amount of water.
Have a row of Karl Forester grass, lovely, but too tall for site, pushed over by wind. Site is in front of stone wall with southern sun. Gets hot in summer, 4b zone cold in winter. Amount of water given is adjustable.

Certified GKH Gardening Expert
Here are some options. One of my favorites under 4 feet is Little Bluestem, a native grass with bluegreen foliage. Several colorful cultivars are available. Since you are in the mountain states region, I included a publication from Colorado State University that oulines grasses for your area.
https://extension.colostate.edu/topic-areas/yard-garden/ornamental-grasses-7-232/