When was the the 4" spacing put in place?
I don’t know, but isn’t it something like 2 3/8" now?
Does it matter?..you could recommend repair for enhanced safety, not injecting code and adhering to Nachi SOP. 
SEE:
SOP
2.2. Exterior
I. The inspector shall inspect:
[INDENT]A. The siding, flashing and trim.
B. All exterior doors, decks, stoops, steps, stairs, porches, railings, eaves, soffits and fascias.
C. **And report as in need of repair any spacings between intermediate balusters, spindles, or rails for steps, stairways, balconies, and railings that permit the passage of an object greater than four inches in diameter. **
D. A representative number of windows.
E. The vegetation, surface drainage and retaining walls when these are likely to adversely affect the structure.
F. And describe the exterior wall covering.
[/INDENT]
information is power.
This might help
**Handrail requirements. **
***UBC 1003.3.3.6 1997 ****“Stairways shall have handrails on each side, and every stairway required to be more than 88 inches (2235 mm) in width shall be provided with not less than one intermediate handrail for each 88 inches (2235 mm) or required width. Intermediate handrails shall be spaced approximately equally across with the entire width of the stairway. *
*EXCEPTIONS: 1. Stairways less than 44 inches (1118 mm) in width or stairways serving one individual dwelling unit in Group R, Division 1 or 3 Occupancy or a Group R, Division 3 congregate residence may have one handrail. *
*2. Private stairways 30 inches (762) or less in height may have a handrail on one side only. *
3. Stairways having less than four risers and serving one individual dwelling unit in Group R, Division 1 or 3, or a Group 4, Division 3 congregate residence or Group U Occupancies need not have handrails.
**4. Baluster Spacing: **The general rule is to install balusters and newel posts between the handrail and flooring materials so that a 4” sphere (the head of an infant) cannot pass through.