Depends on where in Florida - Florida went with a unified Florida Building Code with the 2001 Florida Building Code, which was effective March 1, 2002 (
http://www.floridabuilding.org/fbc/thec ... 2-0315.pdf ).
Prior to that, basically each AHJ had its own code, except Broward County (South Florida Building Code, Broward County Edition) and Miami-Dade County (South Florida Building Code, Miami-Dade Count Edition).
Considering that there are 68 counties in Florida, and each county typically has at least 2-3 towns/cities, 68 - 2 (subtracting for Broward and Miami-Dade) = 66 x 2 = 130 AHJ with each having their own code or own version of a code.
Most of Florida was under one of the editions of the Standard Building Code, but some used the Uniform Building Code, so you can see what a mess it was before the Florida Building Code.
Within reason, as a logical guess, most (if not all) of the editions of the various codes in effect in the 1990s likely had the 'shall not allow a 4 inch sphere to pass' limitation.
Before that, some codes allowed a 6 inch sphere size opening.
A better question would be 'the home is located in ... and was built in ... what was the maximum opening size permitted for guards' ... while that would narrow the potential answer down dramatically, it would still likely not provide enough information for a definitive answer, that could only come with three things: 1) what code was in effect at the time of construction; 2) what edition of that code was in effect; 3) what amendments, if any, had the AHJ made to the code.
As an example, I pulled out the 1988 Standard Building Code, maximum opening size was 6 inches, but it also contained a maximum height of 2 inches for the bottom rail of the guard railing where the balusters did not go all the way down.
The next code I pulled out was the 1994 Standard Building Code, same thing as the 1988.
Next I pulled out the 1997 Standard Building Code, the opening size was reduced to 4 inches from the previous 6 inches.