Distance at bottom of porch railings
The International Code Council says in the International Building Code that a 4" sphere shall not pass through a "guard" for stairs over 30" above finish floor...except for the triangular space formed by the riser, tread and bottom of the guardrail. In which case, it cannot allow a 6" sphere to pass. (There are other exceptions, but none that pertain to your issue.) See Section 1013.4, Exception 2 of the IBC.