For years now, we have been told to use the 4 inch on center rule for our balusters or stainless steel stair cables when installing rail. This article by Shelly Hutchins may change your mind.

..."High Strung Precision was key in creating a continuous four-story stair in this San Francisco remodel. Originating from within a set of kitchen cabinets that also forms the first flight of steps, 3/8-inch stainless steel stair tension cables rise 35 feet to the ceiling, where they're attached to a steel bar. It was painstaking work to get each cable threaded through four stories of aligned holes drilled into tabs at the edge of the steel support pans beneath the vertical-grain Douglas fir treads. The effort, and spacing the cables 2 3/4 inches apart, assured that no one would slip through a gap between the cable and the stair. Each cable has a built-in adjustment knuckle at the top to increase the tension as needed. When fully tightened, the cables cannot be pulled more than the required 4 inches apart. A stirrup extends from the bottom of each tread to minimize gaps between the steps. The overall result is an open stairwell that feels as if it's suspended from the ceiling.Asian Grace Traditional Japanese Tansu cabinets inspired stepped storage underpinning the stainless steel staircase in architect Sandra Vivanco's remodeled Edwardian home. Vivanco customized the size of each stairwell compartment to accommodate a particular function--the subwoofer hides behind a perforated metal screen on the corner while wine discreetly ages in a perfectly square rack facing the dining room. Flat panel doors keep the look clean. Open treads above offset the heaviness of the built-in white oak and bamboo base cabinet. "As the stair travels upward, the elements get lighter and lighter until all that's left is a wood trellis at the top landing," says Vivanco. Thin steel-and-glass railings perpetuate an airy look on the higher steps and allow sunshine from a skylight to trickle down. Vivanco also wanted to highlight structural function, so fine details such as laser-cut support beams beneath the landing reveal exactly where the heaviest gravity loads occur. Great Escape When architect Dan Rockhill's clients requested an exceptional stair leading to their lofty library, he thought of the wonder induced by metal fire escape stairs that fold away just out of reach. When he found out his clients collected old scientific instruments, Rockhill knew that a stairwell contraption reminiscent of a fire escape was the way to go. The whole language of this Lawrence, Kan., house is steel, including the frame, so clear-coated gauged steel stairs fit that vocabulary.